Multicast service providing system, multicast service providing method, information distributor, radio terminal, and radio base station

ABSTRACT

The present invention is configured such that, in a multicast service providing system in which multicast information distribution service is made from an information distributing apparatus to radio terminals present within a service area via a radio section, the radio terminal has service continuation request signal transmitting a service continuation request signal for requesting continuation of the multicast information distribution service to the information distributing apparatus, the information distributing apparatus has service continuation management means managing as to whether or not request for the service continuation has been made by the radio terminal for the multicast information distribution service, and the information distributing apparatus transmits information indicating as to whether or not continuation request has been made for the multicast information distribution service managed by the service continuation management means.

TECHNICAL FIELD

[0001] The present invention relates to a multicast service providingmethod and system, and, in particular, a multicast service providingmethod and system by which, to radio terminals present in a servicearea, multicast information (multicast data) distribution service isperformed via a radio section.

[0002] Furthermore, the present invention relates to an informationdistributing apparatus, a radio terminal and a radio base stationapplied to the above-mentioned multicast service providing system.

BACKGROUND ART

[0003] An IGMP (Internet Group Management Protocol) is known as amulticast communication protocol realizable in an IP network. The IGMPis a protocol for determining whether or not a router sends outmulticast information to a sub-network, for the purpose of avoidingcongestion of a network. An access communication system connected to thenetwork which supports the IGMP should employ a control procedure whichhas an affinity with the IGMP.

[0004] In a communication system according to the IGMP, while the routerdistributes (broadcasts) multicast information to each host in thesub-network, the router broadcasts query periodically. The host whichreceived the query monitors as to whether a response from another hostis received within a random time. When a response from another host isreceived by the host, this host maintains a reception condition formulticast information. However, no response from another host isreceived by the host, this host transmits a response (report) to therouter and the other host so as to continue reception of multicastinformation. Then, when the router receives a response (report) from anyhost in response to the periodically broadcast query, the routercontinues distribution (broadcast) of the multicast information to eachhost.

[0005] Such a processing procedure may be applied to a radio section asit is. In this case, as shown in FIG. 1, a message corresponding to theabove-mentioned query is transmitted to all the radio terminals presentin a service area by a radio base station. The radio terminal which hasreceived the message transmits a request for the service continuation asa response thereto to the radio base station after a random time haselapsed, when desiring continuation of multicast informationdistribution service. When receiving a response to the above-mentionedquery from any radio terminal, the radio base station continues themulticast information distribution service.

[0006] Thus, in case the processing procedure of IGMP is applied to theradio section, when receiving the message corresponding to the queryfrom the radio base station, the radio terminal can transmit a responsethereto to the radio base station. However, it is not possible totransmit another radio terminal in case of IP network. If theabove-mentioned response transmitted from the radio terminal could notbe received by all the other radio terminals which receive multicastinformation in the same service area, the response would be returned tothe radio base station from all the radio terminals which desirecontinuation of receiving the multicast information. Thereby, congestionis likely to occur in the radio section.

[0007] Further, an access to a channel transmitting the above-mentionedrequest for service continuation occurs randomly from a plurality ofradio terminals. However, in case the permissible number of the accessesis limited for this channel, collision occurs when many radio terminalssimultaneously transmit the requests for service continuation by chance.Thereby, as shown in FIG. 1, the request for service continuation doesnot reach the radio base station (see a sign ×). When the radio basestation operates in accordance with a protocol such as IGMP, the radiostation terminates multicast information distribution service after apredetermined time has elapsed without transmission of a request forservice continuation made from any radio terminal after transmitting thequery. Thereby, the service would be interrupted even though there arethe radio terminals which request service of distributing multicastinformation in the service area if the request for service continuationmade from the many radio terminals simultaneously by chance collided asmentioned above.

[0008] Further, recently, verification for a multicast applicationperforming broadcast-like distribution of music or movies via theInternet employing a cable network, or performing cooperative works orremote meeting by a plurality of users has been performed by using IGMPperforming group management of various multicast service.

[0009] On the other hand, through spread of portable telephone terminalssuch as cellular phones, PHS terminals, or the like, or portableinformation terminals such as so-called notebook personal computers, itis expected that a demand of radio provision of multicast serviceincreases. When an PC terminal of an end user of a network and a routerof a sub-network in which this PC terminal exists can operate inaccordance with IGMP, provision of multicast service can be made whetherthe channel is of cable or radio.

[0010] Therefore, a system such as that shown in FIG. 2 can be assumedas a system performing the above-mentioned provision of multicastservice, for example. This example employs a radio base station 220which can operate in accordance with IGPM which acts as an access pointfor a radio LAN. In this case, multicast information A, B and C providedby various servers 251, 252 and 253 via an IP network NW and a router230 is distributed from the radio base station 220.

[0011] For example, when radio terminals 210(1), 210(2) and 210(3)including radio LAN connection communication devices belonging to aservice area Es (LAN) of radio LAN and PC terminals request multicastinformation A, B and C different from each other, the radio base station220 broadcasts all the multicast information A, B and C thus requested.Then, each of the radio terminals 210(1), 210(2) and 210(3) receives allthe multicast information A, B and C. Then, in each radio terminal (PCterminal), necessary multicast information is extracted from thethus-received information to be used.

[0012] In case the radio base station 220 distributes multicastinformation in the service area Es (LAN) of radio LAN, no individualradio channel is set for each radio terminal. However, a data streamincluding the plurality of sets of multicast information broadcast fromthe radio base station 220 mixed therein is received by each radioterminal (cellular phone and PC terminal), and is stored thereby, and,then, only necessary information is extracted therefrom. Therefore, wheneach radio terminal receives a large amount of multicast information, aheavy burden is loaded thereon for buffering it.

[0013] Furthermore, a system such as that shown in FIG. 3 can also beassumed as a system performing the above-mentioned radio provision ofmulticast service, for example. This example performs multicast serviceby using a radio base station 220 connected to a public network NW suchas an existing PDC (Personal Digital Cellular) or PHS. In this case,multicast information A provided from a server 250 via the publicnetwork NW is distributed from the radio base station 220 to respectiveradio terminals 210(1), 210(2) and 210(3) belonging to a service area Esthereof. In distributing the multicast information A, an individualchannel is set between each of the radio terminals 210(1), 210(2) and210(3), and the radio base station 220.

[0014] In such a system in which multicast service is performed by usingthe base station 220 connected to the public network NW, the individualradio channel should be set for each radio terminal even fortransmitting the same multicast information within the same service areaEs. Accordingly, it is not possible to effectively utilize the radioresources.

[0015] Furthermore, a system shown in FIG. 4 can also be assumed forperforming the above-mentioned radio provision of multicast information,for example. This example performs multicast service in an advancedradio calling system (FLEX-TD). In this system, a radio base station 220transmits multicast information provided from various servers 251, 252and 253 via a public network NW by using a radio channel set for eachset of multicast information regardless of whether or not radioterminals which desire the service belonging to a service area (radiozone) Es exist, in a broadcast-like manner. Each of the radio terminals210(1), 210(2) and 210(3) can receive multicast information for whichcontract has been made previously. For example, the radio terminal210(1) which made a contract for multicast information A can receiveonly the multicast information A from among multicast information A, Band C, the radio terminal 210(2) which made a contract for multicastinformation A and C can receive only the multicast information A and C,and the radio terminal 210(3) which made a contract for multicastinformation C can receive only the multicast information C.

[0016] However, in such a system, each radio terminal can receive onlymulticast information for which a contract has been made. However, evenwhen the radio terminal which made a contract of receiving multicastinformation does not exist within the service area Es, the radio basestation 220 should perform continues transmission of the multicastinformation. Thus, it is not possible to effectively utilize the radioresources.

[0017] Furthermore, when information multicast distribution service isrealized in a radio communication system, in particular, in a mobilecommunication system, smooth switching of base station from whichinformation is received by a mobile set is needed when the mobile setwhich is a radio terminal moves across service areas (in case ofhandoff) while receiving the multicast distribution of the information.

[0018] Thus, in case information multicast distribution service isapplied to a radio communication system, various problems should besolved.

DISCLOSURE OF THE INVENTION

[0019] A general object of the present invention is to provide a new anduseful multicast service providing system, method and, informationdistributing apparatus, radio terminal and radio base station by whichthe above-described problems of the prior art can be solved.

[0020] A detailed object of the present invention is to provide amulticast service providing system, method and, information distributingapparatus, radio terminal and radio base station by which radioresources can be effectively utilized, and, also, it becomes possible toproperly perform multicast information distribution service via a radiosection to radio terminals present within a service area.

[0021] The objects of the present invention can be attained byconfiguring a radio terminal to have service continuation requestingmeans transmitting to an information distributing apparatus a servicecontinuation request signal for requesting continuation of multicastinformation distribution service, in a multicast service providingsystem for performing multicast information distribution service to theradio terminal present within a service area via a radio section fromthe information distributing apparatus, and, by configuring theinformation distributing apparatus to have a service continuationmanagement means managing as to whether or not the above-mentionedrequest for service continuation from the radio terminal for themulticast information distribution service has been made, theinformation distributing apparatus transmitting information managed bythe service continuation management means to the radio terminalinformation indicating whether or not the continuation request for themulticast information distribution service has been made.

[0022] In this multicast service providing system, the radio terminalcan determine whether or not the service continuation request signal isto be transmitted thereby, based on the information indicating whetheror not continuation request for the multicast information distributionservice has been made transmitted from the information distributingapparatus. Then, when this information indicating whether or notcontinuation request for the multicast information distribution servicehas been made indicates a state that this continuation request has notbeen made, the service continuation requesting means of the radioterminal can transmit the service continuation request signal to theinformation distributing apparatus.

[0023] Further, the information distributing apparatus can determinewhether or not the multicast information distribution service should becontinued, based on the management state of the service continuationmanagement means.

[0024] In the above-mentioned multicast information distributing system,the service management means may be configured to have management stateresetting means resetting the management state into a state that norequest for continuation of the multicast information distributionservice has been made from the radio terminal.

[0025] In this multicast service providing system, the informationdistributing apparatus can positively determines whether or not theradio terminal requesting the multicast information distribution serviceis present within the service area. That is, when the servicecontinuation request signal is transmitted from the radio terminal afterthe management state of the service management means is reset to a statethat no request for continuation of the multicast informationdistribution service has been made, the information distributingapparatus can determine that the radio terminal which requests themulticast information distribution service is present within the servicearea.

[0026] Furthermore, in the above-mentioned multicast informationdistributing system, the above-mentioned management state resettingmeans may be configured to perform the resetting operation for themanagement state every predetermined period.

[0027] In this multicast information distributing system, theabove-mentioned positive determination made by the informationdistribution apparatus can be made periodically.

[0028] Furthermore, in the above-mentioned multicast informationdistributing system, the radio terminal may have continuation requestcontrol means performing control such that, when the informationindicating whether or not continuation request for the multicastinformation distribution service has been made transmitted from theinformation distribution apparatus indicates a state that the requesthas not been made, the service continuation requesting means transmitsthe service continuation request signal to the information distributingapparatus.

[0029] In this multicast service providing system, the request formulticast information distribution service continuation can be properlytransmitted according to the management state of the informationdistributing apparatus.

[0030] Further, in the above-mentioned multicast service providingsystem, the above-mentioned continuation request control means may beconfigured to control the above-mentioned service continuationrequesting means so that the above-mentioned service continuationrequest signal is transmitted after a first random time has elapsedsince the information indicating the state that no continuation requesthas been made for the multicast information distribution service wasreceived.

[0031] In this multicast service providing system, in case a pluralityof radio terminals requesting continuation of the multicast informationdistribution service exist within the service area, it is possible toreduce a probability of collision of the service continuation requestsignals transmitted to the information distributing apparatus from therespective radio terminals. That is, in the case where the plurality ofradio terminals requesting continuation of the multicast informationdistribution service exist within the service area, each radio terminaltransmits the above-mentioned service continuation request signal at arandom timing after receiving from the information distributingapparatus the information indicating the state that no continuationrequest has been made for the multicast information distributionservice. Thereby, a probability that each radio terminal simultaneouslytransmits the service continuation request signal to the informationdistributing apparatus is made smaller.

[0032] Further, in the above-mentioned multicast service providingsystem, the radio terminal may be configured to have re-transmissioncontrol means performing a control such that the above-mentioned servicecontinuation requesting means re-transmits the service continuationrequest signal to the information distributing apparatus when theinformation indicating as to whether or not continuation request hasbeen made for the multicast information distribution service has notbeen changed into information indicating that the request has been madewithin a predetermined time after the service continuation requestingmeans transmute the service continuation request signal.

[0033] In this multicast service providing method, even when the servicecontinuation request signal transmitted from the radio terminal has notbeen received by the information distributing apparatus by some reason,it is possible to prevent the multicast information distribution servicefrom being terminated for the radio terminal.

[0034] Further, in the above-mentioned multicast service providingsystem, the above-mentioned re-transmission control means may beconfigured to control the above-mentioned service continuationrequesting means so that the above-mentioned service continuationrequest signal may be transmitted again after a second random time haselapsed after the above-mentioned predetermined time had elapsed sincethe service continuation requesting means transmitted the servicecontinuation request signal.

[0035] In this multicast service providing system, in case a pluralityof radio terminals exist within the service area, it is possible toprevent the service continuation request signals re-transmitted from therespective radio terminals from colliding.

[0036] Further, in the above-mentioned multicast service providingsystem, a setting may be made such that a time elapsing since theabove-mentioned management state resetting means reset the managementstate of the service request management means into a state that norequest for continuation of the multicast information distributionservice until the above-mentioned service continuation request meansre-transmits the service continuation request signal may be shorter thana time elapsing since the above-mentioned management state was reset toa state that continuation for a service has not been made from the radioterminal for the multicast information distribution service untilresetting is made subsequently.

[0037] In this multicast service providing system, it is possible topositively change the management state of the information distributingapparatus by the above-mentioned service continuation request signalre-transmitted.

[0038] Further, in the above-mentioned multicast service providingsystem, the above-mentioned service continuation management means may beconfigured to manage as to whether or not continuation request has beenmade for the multicast information distribution service by using a flagwhich is made to be ON when the service continuation request signal isreceived from the radio terminal.

[0039] In this multicast service providing system, it is possible toeasily perform management performed by the service continuationmanagement means in the above-mentioned information distributingapparatus.

[0040] Further, the above-mentioned object of the present invention maybe attained as a result of the above-mentioned information distributingapparatus which performs multicast information distribution service toradio terminals present within a service area via a radio section havingservice continuation management means managing as to whether or notrequest for service continuation from the radio terminals for themulticast information distribution service has been made, and beingconfigured so that information managed by the service continuationmanagement means indicating whether or not the continuation request forthe multicast information distribution service has been made may betransmitted to the radio terminals.

[0041] Further, the above-mentioned object of the present invention maybe attained as a result of a radio terminal which receives multicastinformation distribution service from an information distributingapparatus via a radio section having service continuation requestingmeans transmitting a service continuation signal for requestingprovision of the multicast information distribution service to theinformation distributing apparatus, and continuation request controlmeans performing control such that the above-mentioned servicecontinuation requesting means may transmit the service continuationrequest signal to the information distributing apparatus wheninformation, transmitted from the information distributing apparatusmanaging as to whether or not a request for service continuation hasbeen made by the radio terminal for the multicast informationdistribution service, indicating whether or not service continuationrequest has been made from the radio terminal for the multicastinformation distribution service indicates a state that the request hasnot been made.

[0042] Further, the above-mentioned object of the present invention maybe attained as a result of, in a multicast service providing method bywhich multicast information distribution service is performed by aninformation distributing apparatus via a radio section on a radioterminal present in a service area thereof, the information distributingapparatus informing all the radio terminals present in the service areaof information for identifying multicast information on distributionservice and radio channels used for the distribution of the multicastinformation by using a predetermined radio channel, and the respectiveradio terminals within the service area receiving the multicastinformation distribution service from the information distributingapparatus via the thus-informed radio channels.

[0043] In this multicast service providing method, the radio terminalreceives the multicast information distribution service by using theradio channel informed of by the information distributing apparatus.Accordingly, the radio channel used for the multicast informationdistribution service can be made to be only the radio channel informedof by the information distributing apparatus, and, also, thedistribution service made via the radio channel can be made to be onlyof multicast information corresponding to the informed radio channel.

[0044] The above-mentioned information for identifying the multicastinformation informed of by the information distributing apparatus is notparticularly limited as long as it can be used for identifying themulticast information provided based on this information at the radioterminal, and, may be information identifying a type of the multicastinformation, the channel providing it (multicast group address),information identifying a server which originally provides theinformation, a title of the multicast information, or the like.

[0045] Further, in the above-mentioned multicast service providingmethod, the information distributing apparatus may be configured tomanage correspondence relationship between the information foridentifying multicast information on distribution service and the radiochannels used for distributing the multicast information, informs allthe radio terminals present within the service area managementinformation indicating the correspondence relationship by using theabove-mentioned predetermined radio channel, and, each of the radioterminals present within the service area receives the desired multicastinformation distribution services by using the corresponding radiochannels based on the management information.

[0046] In this multicast service providing method, it is possible toinform the radio terminals of relationship between the information foridentifying a plurality of sets of multicast information on distributionservice and the radio channels used for distributing the respective setsof multicast information without error.

[0047] Further, in the above-mentioned multicast service providingmethod, the radio terminal transmits a service request signal forrequesting distribution service for multicast information not includedin the above-mentioned management information, and, when receiving theabove-mentioned service request signal from the radio terminal, theinformation distributing apparatus adds to the above-mentionedmanagement information correspondence relationship between informationfor identifying the multicast information concerning this request and aradio channel used for distributing this multicast information, and,also, starts distribution service of the multicast informationconcerning the request by using this radio channel.

[0048] In this multicast service providing method, the radio terminalcan receive distribution service of the new multicast information otherthan the multicast information which is currently on distributionservice.

[0049] Further, the above-mentioned object of the present invention maybe attained as a result of, in an information distributing apparatuswhich performs multicast information distribution service for radioterminals present within a service area via a radio section, aconfiguration being made such as to have informing control means forinforming of all the radio terminals present within the service areainformation for identifying multicast information on distributionservice and radio channels used for distributing the multicastinformation by using a predetermined radio channel, and, such that eachof the radio terminals present within the service area can receive themulticast information distribution service via the thus-informed radiochannels.

[0050] Further, the above-mentioned object of the present invention maybe attained as a result of, in a radio terminal which receives multicastinformation distribution service via a radio section from an informationdistributing apparatus, a configuration being made such as to haveservice state reception control means receiving from the informationdistributing apparatus management information indicating correspondencerelationship between information for identifying multicast informationon distribution service and radio channels used for distributing themulticast information, such as to receive from the informationdistributing apparatus the desired multicast information distributionservice via the corresponding radio channel based on the managementinformation received by the service state reception control means.

[0051] Further, the above-mentioned object of the present invention maybe attained as a result of, in a multicast service providing method bywhich multicast data is distributed to radio terminals from a radio basestation, a configuration being made such that the radio terminalreceiving the multicast data may transmit a distribution request for themulticast data to the radio base station on movement destination in casea necessity arises such that the other party of communication beswitched from the radio base station on movement origin to the radiobase station on movement destination, the radio base station on movementdestination may transmit information of a radio channel used fordistributing the multicast data to the above-mentioned radio terminal inresponse to the distribution request given from the above-mentionedradio terminal, the radio terminal may switch the radio channel based onthe information of the radio channel given from the above-mentionedradio base station on movement destination so as to receive themulticast data from the radio base station on movement destination.

[0052] In this multicast service providing method, when a necessityarises such that the other party of communication be switched from theradio base station on movement origin to the radio base station onmovement destination, the radio terminal should merely transmit adistribution request to the radio base station on movement destination,and, thereby, in response to the distribution request, information ofthe radio channel used for distributing the multicast data istransmitted from the radio base station on movement destination.Therefore, the radio terminal receives the multicast data by switchingthe radio channel based on this information of the radio channel.Accordingly, it is possible to switch the radio base station acting asthe other party of communication, rapidly than in a case, as in theprior art, the radio terminal again receives a perch channel from theradio base station on movement destination, further receives a multicastmanagement table based on information included in the perch channel,then, recognizes a radio channel to be used for transmission from themanagement table, and, thus, receives the multicast data.

[0053] Further, in the above-mentioned multicast service providingmethod, a configuration may be made such that the radio terminal maytransmit the above-mentioned distribution request for the multicast datato the radio base station on movement origin, and the above-mentioneddistribution request for the multicast data may be transmitted to theradio base station on movement destination via this radio base stationon movement origin.

[0054] In this multicast service providing method, the distributionrequest given from the radio terminal can be made to be positivelyreceived by the radio base station on movement destination.

[0055] Further, in the above-mentioned multicast service providingmethod, a configuration may be made such that, in case the radioterminal cannot receive information of a radio channel to be used fordistributing multicast data within a predetermined time aftertransmitting the distribution request, the radio terminal may transmitthe distributing request again.

[0056] Also by this multicast service providing method, the distributionrequest given from the radio terminal can be made to be positivelyreceived by the radio base station on movement destination.

[0057] Further, in the above-mentioned multicast service providingmethod, a configuration may be made such that the radio terminal maytransmit the distribution request several times.

[0058] Also by this multicast service providing method, the distributionrequest given from the radio terminal can be made to be positivelyreceived by the radio base station on movement destination.

[0059] Further, in the above-mentioned multicast service providingmethod, a configuration may be made such that the radio base station onmovement origin may obtain information concerning multicast datadistributed by an adjacent radio base station, and transmit theinformation concerning the multicast data, and, in case the radioterminal determines that the radio base station on movement destinationdoes not distribute the desired multicast data based on the informationconcerning the multicast data thus given from the radio base station onmovement origin, the radio terminal may transmit the distributionrequest to the radio base station on movement destination or the radiobase station on movement origin so that the above-mentioned distributionrequest may be received by the radio base station on movementdestination.

[0060] In this multicast service providing method, the multicast datawhich the radio base station on movement destination distributes is madeto be previously recognized by the radio terminal, and, when the radiobase station on movement destination distributes the desired multicastdata, the multicast data can be received by the radio terminalimmediately without transmission of distribution request. Accordingly,reception of the multicast data at a time of movement can be maderapidly.

[0061] Further, for example, the information concerning the multicastdata includes information for identifying the multicast data ondistribution, and information of radio channels by which the multicastinformation is distributed.

[0062] Further, the object of the present invention may be attained in aradio base station which transmits multicast data to a radio terminal,as a result of a configuration being made such as to have distributionrequest receiving means receiving a distribution request for multicastdata transmitted from the radio terminal, and radio channel informationtransmitting means transmitting information of a radio channel used fordistributing multicast data in response to the distributing request forthe multicast data given from the radio terminal.

[0063] In this case, a configuration may be made such that theabove-mentioned distribution request receiving means may receive thedistribution request for the multicast data transmitted from the radioterminal via another radio base station. Further, a configuration may bemade such that the above-mentioned distribution request receiving meansmay have information obtaining means obtaining information concerningmulticast data transmitted from a radio base station adjacent to the ownstation, and multicast information transmitting means transmitting theinformation concerning the multicast information.

[0064] Further, the above-mentioned object of the present invention maybe attained, as a result of, in a radio terminal which receivesmulticast data distributed by a radio base station, a configurationbeing made such as to have distribution request transmitting meanstransmitting a distribution request for multicast data to the radio basestation on movement destination in case a necessity arises such that theother party of communication be switched from the radio base station onmovement origin to the radio base station on movement destination, andmulticast data reception control means switching the radio base stationbased on information concerning a radio channel used for distributingthe multicast data transmitted from the radio base station on movementdestination in response to the distribution request so as to receive themulticast data distributed from the radio base station on movementdestination.

[0065] In this case, a configuration may be made such that theabove-mentioned distribution request transmitting means may transmit theabove-mentioned distribution request for the multicast data to the radiobase station on movement origin, and, then, the above-mentioneddistribution request for the multicast data may be transmitted to theradio base station on movement destination via the radio base station onmovement origin. Further, a configuration may be made such that theabove-mentioned distribution request transmitting means may transmit thedistribution request again in case information concerning a radiochannel to be used for distributing the multicast data cannot bereceived thereby. Further, a configuration may be made such that theabove-mentioned distribution request transmitting means may transmit thedistribution request several times. Further, a configuration may be madesuch that the above-mentioned distribution request transmitting meansmay transmit the distribution request to the radio base station onmovement destination or the radio base station on destination origin sothat the above-mentioned distribution request for the multicast data maybe received by the radio base station on movement destination in casethe distribution request transmitting means determines that the radiobase station on movement destination does not distribute the desiredmulticast data based on information, received from the radio basestation on movement origin, concerning multicast information distributedfrom the radio base station adjacent to the station.

[0066] Other objects, features, and advantages of the present inventionmay become apparent from the following detailed description madetogether with attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0067]FIG. 1 is a sequence diagram showing a communication procedurebetween a radio base station and a radio terminal in a multicast serviceproviding system in the prior art.

[0068]FIG. 2 is a diagram showing a first example of a multicast serviceproviding system in the prior art applied to a radio section.

[0069]FIG. 3 is a diagram showing a second example of a multicastservice providing system in the prior art applied to a radio section.

[0070]FIG. 4 is a diagram showing a third example of a multicast serviceproviding system in the prior art applied to a radio section.

[0071]FIG. 5 is a block diagram showing an example of a basicconfiguration of a multicast service providing system in a firstembodiment of the present invention.

[0072]FIG. 6 is a block diagram showing an example of a configuration ofa radio base station in the system shown in FIG. 5.

[0073]FIG. 7 is a diagram showing an example of a management table whichthe radio base station in the multicast service providing system has.

[0074]FIG. 8 is a diagram showing an example of state transition in theradio base station.

[0075]FIG. 9 is a flow chart which shows an example of a processingprocedure in the radio base station.

[0076]FIG. 10 is a flow chart which shows an example of a processingprocedure in each radio terminal.

[0077]FIG. 11 is a sequence diagram showing an example (number 1) of acommunication procedure between the radio base station and the radioterminal.

[0078]FIG. 12 is a sequence diagram showing an example (number 2) of thecommunication procedure between the radio base station and the radioterminal.

[0079]FIG. 13 is a diagram showing an example of a configuration of amulticast information channel table for managing multicast informationchannels by which service provision is made from the radio base station.

[0080]FIG. 14 is a flow chart which shows an example of a processingprocedure in the radio base station which performs service provision.

[0081]FIG. 15 is a flow chart which shows an example of a processingprocedure in the radio terminal which receives service provision fromthe radio base station.

[0082]FIG. 16 is a diagram showing an example of a relationship betweenvarious information transmitted by the radio base station in case ofperforming multicast service, and transmission channels therefor.

[0083]FIG. 17 is a flow chart which shows an example of a processingprocedure for the radio base station to determine continuation ofservice with the radio terminal.

[0084]FIG. 18 is a diagram showing an example of a configuration of amulticast information channel table in case of transmitting a servicecontinuation signal by a same channel as that for the multicastinformation channel table.

[0085]FIG. 19 is a flow chart which shows an example of a processingprocedure for informing the radio base station that the radio terminaldesires continuation of service.

[0086]FIG. 20 is a flow chart which shows an example of a processingprocedure at a time of the radio terminal terminating reception ofmulticast information.

[0087]FIG. 21 is a diagram showing an example of a basic configurationof a system to which multicast service providing method in a thirdembodiment of the present invention is applied.

[0088]FIG. 22 is a conceptual diagram of operation in a first example ofoperation.

[0089]FIG. 23 is a flow of operation in the first example of operation.

[0090]FIG. 24 is a conceptual diagram of operation in a second exampleof operation.

[0091]FIG. 25 is a diagram showing an example of a multicast managementtable used in the second example of operation.

[0092]FIG. 26 is a flow of operation in the second example of operation.

BEST MODE FOR CARRYING OUT THE INVENTION

[0093] Hereafter, embodiments of the present invention will be describedbased on the drawings.

[0094] First, a multicast service providing system in a first embodimentof the present invention will be described.

[0095] This system is configured as shown in FIG. 5.

[0096] In FIG. 5, a radio base station 20 broadcasts information A(henceforth, referred to as multicast information) provided, forexample, through a public network, etc. to a service area Es. Radioterminals 10 (cellular phones, personal computers which have acommunication function, or the like) present in this service area Es canreceive the multicast information A broadcast by the radio base station20.

[0097] The radio base station 20 is configured as shown in FIG. 6.

[0098] In FIG. 6, this radio base station 20 has atransmitting/receiving unit 21, a multicast information storing part 22,a network control part 23, and an information distribution control part24. The transmitting/receiving unit 21 performs radio communicationswith each radio terminal 10 present in the service area Es. Themulticast information storing part 22 stores multicast information to bedistributed which the network control part 23 receives from the network.The information distribution control part 24 performs control fordistributing the multicast information stored in the multicastinformation storing part 22 through the transmitting/receiving unit 21to each radio terminal 10 present in the service area Es.

[0099] The information distribution control part 24 has a managementtable as shown in FIG. 7. This management table is used for managing asto whether requests for continuation have been made for variousdistribution services of multicast information. In FIG. 7, whencontinuation requests for various distribution services (multicastgroups) of multicast information are made, flags (#1, #2, #3, and #4,referred to also as received flags) corresponding to the services aremade to be ON, and, when no continuation requests for distributionservices of multicast information are made, the flags corresponding tothe services are made to be OFF.

[0100] Moreover, the radio base station 20 (information distributioncontrol part 24) resets (makes to be OFF) all the flags in theabove-mentioned management table at a predetermined timing. Then, thestate (ON/OFF) of each flag managed in the above-mentioned managementtable is broadcast from the radio base station immediately after theflag reset timing (after a predetermined time). Such a state of theradio base station 20 is managed by a reset state flag of a reset stateitem in the above-mentioned management table. Namely, the reset stateflag is made to be ON (ON) together with resetting (OFF) of the flagscorresponding to the above-mentioned various distribution services, andthe reset state flag is made to be OFF (OFF) from a timing immediatelyafter that (after a predetermined time) at which each flag state isbroadcast until all the flags are subsequently reset. Therefore, thestate of the radio base station 20 changes by turn between a state wherethe reset state flag is made to be ON (ON), and a state where the resetstate flag is made to be OFF (OFF), as shown in FIG. 8.

[0101] In case the various distribution services of multicastinformation are provided from the radio base station 20, processing isperformed by the radio base station 20 according to a procedure shown inFIG. 9, and the radio terminal 10 which receives the multicastinformation performs processing according to a procedure shown in FIG.10.

[0102] In FIG. 9, the radio base station 20 resets the flags (#1, #2,#3, #4) corresponding to all the distribution services in theabove-mentioned management table (see FIG. 6) at a predetermined timing(S1). The reset state flag is made to be ON at this time. The resetstate flag is made to be OFF immediately after that (after thepredetermined time). Thus, a signal showing the reset state flag whichis made to be ON or made to be OFF is broadcast using a differentchannel from a channel used for the various distribution services ofmulticast information. Moreover, when the above-mentioned reset stateflag is switched to OFF, a signal showing the states of the flagscorresponding to the various distribution services managed in theabove-mentioned management table are broadcast using the predeterminedchannel.

[0103] When the flags corresponding to all distribution services arereset as mentioned above, it is determined (S3) whether it is thesubsequent timing of flag reset, while it is determined (S2) whether theradio base station 20 has received a service continuation request signal(which will be described later in details) from any one of the radioterminals 10. In this process, in response to reception of the servicecontinuation request signal from any one of the radio terminals 10 (YESat S2), the flag corresponding to the relevant service is made to be ONin the management table (S4).

[0104] When it becomes the subsequent timing of flag reset (YES at S3),it is determined whether the flag corresponding to each service of themanagement table has been made to be ON (S5). Then, the distributionservice corresponding to the flag which has been made to be ON iscontinued, the distribution service corresponding to the flag which hasbeen made to be OFF is terminated (S6), and then, the series ofprocessing is finished (YES at S5). Then, again, while all the flagscorresponding to distribution services of multicast information arereset, and, also, the series of processing is started from theprocessing (S1) at which the reset state flag is made ON.

[0105] Each radio terminal 10 which receives distribution service ofmulticast information performs processing according to a procedure shownin FIG. 10, while the radio base station 20 performs the above-describeprocessing and provides the various distribution services of multicastinformation.

[0106] In FIG. 10, the radio terminal 10 receives a signal indicatingthe reset state flag broadcast by using the predetermined channel, and,in this process, it determines (S12) whether or not this reset stateflag is made to be ON, i.e., whether the flags corresponding to themulticast information distribution services are reset. When it isdetermined that the reset state flag is made to be ON (YES at S12), arandom time R1 is calculated based on a random number generated by arandom number generator (S13). Then, a timer in which the random time R1is set starts operation. After that, it is determined (S14) whether ornot the timer has reached the random time R1 which is the set timethereof.

[0107] When the timer has reached the time R1 (time out), a signalshowing the flag corresponding to the multicast information distributionservice which the radio base station 20 broadcasts is received from thetiming at which the reset state flag is made to be OFF, and it isdetermined whether the flag corresponding to the service which the radioterminal 10 receives is made to be ON (S15). There, when the relevantflag (received flag) is made to be OFF (NO at S15), as management ismade by the radio base station 20 such that it is recognized that theservice continuation signal for the corresponding service has not beenreceived, the radio terminal 10 transmits the service continuationsignal to the radio base station 20 in case it desires continuation ofthe service (S16).

[0108] Then, Twait which is a random time is calculated (S17), andreception of the signal showing the states of the flags corresponding tothe various multicast information distribution services broadcast fromthe radio base station 20 is started (S18). Then, it is determinedwhether or not the random time Twait has been reached (S19), while it isdetermined whether the flag (received flag) corresponding to thedistribution service which the radio terminal 10 receives is made to beON (S20). There, in case the flag corresponding to the relevant servicehas been already made to be ON (see the processing at S2 and S4 in FIG.9) in response to the service continuation request signal transmittedfrom the relevant radio terminal 10 as mentioned above (see theprocessing in S16), the radio terminal 10 finishes the series ofprocessing, and repeats the processing of determining whether or not thereset state flag is made to be ON (the flags corresponding to theservices are made to be OFF) again (S11, S12).

[0109] In a condition where the random time R1 has elapsed from thetiming of detection of ON of the reset state flag, when the flagcorresponding to the relevant service has been already made to be ON inthe radio base station 20 in response to the service continuationrequest signal from another radio terminal 10 which receives the service(YES at S15), the radio terminal 10 finishes the series of processing,and repeats the processing of determining whether or not the reset stateflag is made to be ON (the flags corresponding to the services are madeto be OFF) again (S11, S12).

[0110] As a result of the processing being performed in accordance withthe above-described procedure by the radio base station 20 and radioterminals 10, a communication according to a sequence shown in FIG. 11is performed between the radio base station 20 and the radio terminals10, for example.

[0111] In the process in which the multicast information distributionservice is made on the radio terminal 10 by the radio base station 20,when the reset state flag is made to be ON by the radio base station 20({circle over (1)}), that is, the flags (received flags) correspondingto the respective services are made to be OFF, the signal indicating theflags corresponding to the various services are transmitted to the radioterminal 10 by the radio base station 20 at predetermined timingrepeatedly (see broken line in FIG. 11). In this process, when receivingthe signal indicating the reset state flag having the ON state (theflags corresponding to the services are made to be OFF) from the radiobase station 20 ({circle over (1)}), the radio terminal 10 transmits theservice continuation request signal to the radio base station 20 afterelapse of a random time R1 ({circle over (2)}). Then, the radio terminal10 monitors as to whether the flag corresponding to the relevant serviceis made to be ON during a random time Twait.

[0112] When receiving the above-mentioned service continuation requestsignal from the relevant radio terminal 10 or another radio terminal 10,the radio base station 20 makes the flag corresponding to the service tobe ON. When detecting that the flag corresponding to the service hasbeen made to be ON ({circle over (1)}), the radio terminal 10 monitorsthe reset state flag being made to be ON ({circle over (4)}). Then, eachtime the reset state flag is made to be ON, communication is performedbetween the radio base station 20 and radio terminal 10 in accordancewith the above-mentioned procedure.

[0113] By the above-mentioned processing, the radio base station 20resets (makes to be OFF) the flags corresponding to the variousmulticast information distribution services at predetermined timing,and, also, after the resetting, when receiving the service continuationrequest signal from any radio terminal 10, the radio base station 20makes the flag corresponding to the service to be ON. Accordingly, theradio base station 20 can determine continuation and termination of theservices based on the flags corresponding to the respective servicesmanaged by the management table (see FIG. 7) without managing the numberof the radio terminals 20 which receive the multicast informationdistribution services in the service area Es, or the like.

[0114] Moreover, each radio terminal 10 can check whether another radioterminal 10 which requires the same service as that of itself exists inthe same service area Es by monitoring the states of the flags (receivedflags) corresponding to the respective services broadcast from the radiobase station 20. Then, when the relevant flag is made to be OFF, theradio terminal 10 which desires the service transmits the servicecontinuation request signal to the radio base station 20, as recognizingthat no other radio terminal 10 which requires the same service as thatof itself exists. On the other hand, when the relevant flag is made tobe ON, the radio terminal 10 that desires the service does not transmitthe service continuation request signal, as recognizing that anotherradio terminal 10 which requires the same service as that of itselfexists.

[0115] Thus, the service continuation request signals are nottransmitted from a plurality of radio terminals 10, respectively, whichdesire the same service in the same service area Es, and thus congestionis not likely to occur in the radio section.

[0116] In case the service continuation request signal is transmittedfrom the radio terminal 10 which processes according to theabove-described procedure (see FIG. 10), the service continuationrequest signal may be unable to be received by the radio base station 20due to interference of signals, collision with the service continuationrequest signal simultaneously transmitted by chance from another radioterminal, or the like in the radio channel. In such a case, as will bedescribed below, the radio terminal 10 performs re-transmission of theservice continuation request signal.

[0117] After transmitting the service continuation request signal to theradio base station 20 (S16), the radio terminal 10 which processes inaccordance with the above-described procedure shown in the FIG. 10further calculates a random time R2 again (S21) after a random timeTwait has elapsed, in case the OFF state of the flag corresponding tothe service received from the radio base station 20 is maintained duringthe random time Twait (NO at S19, and also, YES at S20). Then, after therandom time R2 elapsed (YES at S22), it determines again whether or notth flags (received flags) corresponding to the services broadcast by theradio base station 20 are made to be ON (S23). Then, when the flagcorresponding to the service has the ON state, the radio terminal 10finishes the series of processing while receiving the multicastinformation distribution service broadcast by the radio base station 20.

[0118] The radio terminal 10 re-transmits the service continuationrequest signal, as recognizing that the service continuation requestsignal (see S16) transmitted last time has not been received by theradio base station 20, when the flag corresponding to the servicereceived has the OFF state (S24). Then, after finishing re-transmittingof the service continuation request signal, the radio terminal 10finishes the series of processing.

[0119] Between the radio base station 20 and the radio terminal 10, as aresult of the processing according to the above-described procedurebeing performed by he radio terminal 10, communication according to asequence shown in FIG. 12 is performed.

[0120] In the process in which the radio base station 20 is performingmulticast information distribution service on the radio terminal 10,when the reset state flag is made to be ON by the radio base station 20({circle over (1)}), same as in the case described above (see FIG. 11),that is, when the flags corresponding to the respective services(received flags) are made to be OFF, the signal indicating the states ofthe flags corresponding to the respective services is repeatedlytransmitted to the radio terminal 10 from the radio base station 20 atpredetermined timing (see the broken line in FIG. 12). In this process,when receiving the signal indicating the reset state flag having the ONstate (the flags corresponding to the services are made to be OFF)({circle over (1)}), the radio terminal 10 transmits the service requestcontinuation signal to the radio base station 20 after elapse of arandom time R1 ({circle over (2)}). Then, the radio terminal 10monitors, during a random time Twait, whether the flag corresponding tothe relevant service is made to be ON.

[0121] In case the above-mentioned service request signal is notreceived by the radio base station 20 by some reason such as signalinterference or collision (× mark), and the flag corresponding to theservice given from the radio base station 20 is not made to be ON duringthe random time Twait, the radio terminal 10 re-transmits the servicerequest continuation signal ({circle over (3)}) after further elapse ofthe random time R2 after elapse of the random time Twait. When the flagcorresponding to the relevant service managed by the radio base station20 is made to have the ON state by this service continuation requestsignal re-transmitted by the radio terminal 10, after receiving thisflag in the ON state ({circle over (4)}), the radio terminal 10 monitorsthat the reset state flag is made to be ON ({circle over (5)}).

[0122] Since the service continuation request signal is re-transmittedfrom the radio terminal 10 when the service continuation request signaltransmitted from the radio terminal 10 is not received by some reason bythe radio base station 20, the flag corresponding to the relevantservice managed by the radio base station 20 can be positively made intothe ON state.

[0123] In the above-mentioned system, relationship between the resetperiod Treset, that is, the period at which the flags corresponding toall the multicast information distribution services are reset, theabove-mentioned random times R1 and R2, and the random time Twait areset as follows, for example.

[0124] When making the random time Twait constant, it is set as follows:

Twait=Treset−R 1max−R 2max  (1)

[0125] Moreover, when making the random time Twait variable, it is setas follows:

Twait=(Treset−R 1max)×RATE  (2)

[0126] When making the Twait constant and applying the same value forevery Treset cycle, the maximum time of Twait is calculated according tothe above-mentioned formula (1). Accordingly, calculation of Twait isrealizable by setting up a value smaller than Treset−R1 as the maximum(R2max) of R2, and then using the above-mentioned formula (1).

[0127] Moreover, when making the Twait variable and changing it forevery Treset, it is also possible to use a value acquired by multiplying(Treset−R1) by a fixed coefficient RATE. In this case, since R1 is arandom value, Twait can be made variable. As RATE becomes larger in theabove-mentioned formula (2), the maximum of R2 becomes smaller, andprobability of collision at the time of re-transmitting becomes higher.However, since the probability that the service continuation requestsignal from another radio terminal which receives the same multicastinformation distribution service is transmitted becomes higher, there isan advantage that the necessity of re-transmission becomes lower.

[0128] On the other hand, when RATE is smaller in the above-mentionedformula (2), the probability of collision with the service continuationrequest signal from another radio terminal which receives the samemulticast information distribution service becomes higher. However,since R2 can be made larger, there is an advantage that the probabilityof the collision with the service continuation request signal fromanother radio terminal which receives the same multicast informationdistribution service becomes lower, at the time of re-transmitting theservice continuation request signal.

[0129] Moreover, the probability of collision of the servicecontinuation request signals can be reduced by making RATE in theabove-mentioned formula (2) have a unique value for every radioterminal.

[0130] Although, in the above-mentioned example, re-transmitting of theservice continuation request signal is made once, it is easilyrealizable to change respective parameters so that re-transmitting ofthe service continuation request signal may be performed several times.

[0131] Namely, Twait and R2max may be set such that:

Treset=A×(Twait+R 2max)  (3)

[0132] ‘A’ is an integer and expresses the number of times ofretransmissions.

[0133] As, for the radio base station connected to the network whichsupports the version 1 of IGMP, the maximum time for which an IGMProuter waits for a report from a host is 10 seconds, and, thereby, it isreasonable to set the Treset period of the radio base station equal toor shorter than 10 seconds. Since the maximum time for which the IGMProuter waits for the report from the host is specified in a message inthe case of the network corresponding to the version 2, it is reasonableto set the Treset period to be equal to or shorter than the specifiedtime. However, as for this Treset period, it is also possible to set itregardless of the parameter of IGMP, and, then, for the router, theradio base station may perform operation in accordance with IGMP.Furthermore, in case multicast information distribution service in anetwork which does not support IGMP is carried out, this Treset periodmay be set as a value original for the radio base station.

[0134] In each above-mentioned example, the radio base station acts asan information distributing apparatus.

[0135] A second embodiment of the present invention will now bedescribed.

[0136] In this embodiment, a radio channel used for distribution of eachmulticast information is informed of to each radio terminal of a servicearea. Thereby, each radio terminal can receive the desired multicastinformation by using the radio channel corresponding to the multicastinformation.

[0137] A basic configuration of a multicast service providing system inthe second embodiment is configured as shown in FIG. 5, same as theabove-described embodiment, and, also, a radio base station isconfigured as shown in FIG. 6.

[0138] In the present invention embodiment, an information distributioncontrol part 24 manages multicast service by a multicast informationchannel table as shown in FIG. 13. In this multicast information channeltable, relationship between multicast group addresses for identifyingmulticast information channels for performing distribution services,transmission channels used for transmitting the multicast informationcorresponding to the respective multicast information channels, andinformation channel numbers managed by a radio base station 20 isdescribed.

[0139] The multicast group addresses for identifying the multicastinformation channels for providing the distribution services are usedfor identifying the contents of distribution services, and, are the sameas multicast addresses, for example, used by IGMP (Internet GroupManagement Protocol) which is the multicast protocol of IP (InternetProtocol) network. However, in case the radio base station 20 isconnected with a network other than the IP network, any addresses whichcan be used for identifying servers and the networks providing themulticast services can be used as the multicast group addresses.

[0140] The transmission channels are expressed by channel identifiers bywhich the radio base station 20 and each radio terminal 10 can use foridentification, and indicate channels for transmitting the multicastinformation corresponding to the multicast information channels. In casethe access type is, for example, a time division multiple access type(TDMA), channel identifiers include time slot numbers in radio framesand frequency numbers. There may also be a system which requires neitherfrequencies nor time slots for identifying the radio channels, and asystem which requires another identifiers. In such a case, any type ofidentifiers which are used for identifying the radio channels accordingto an identification method suitable for the system may be used.

[0141] In case transmission channels are changed according to asituation of channel allocation in a radio section between the radiobase station 20 and each radio terminal, the item of the transmissionchannel of this multicast information channel table is updated.

[0142] An information channel number is a number which the radio basestation 20 gives for a multicast group address. The information channelnumbers are numbers unique only to the service area Es of the radio basestation 20, and, when the radio terminal 10 moves to another servicearea by handoff, etc., and connection switching to another radio basestation is made, information channel numbers managed by the newlyconnected radio base station are used.

[0143] These information channel numbers correspond to the multicastgroup addresses, as described above, and may be used as the multicastinformation channels between the radio base station 20 and each radioterminal 10, i.e., simple identifiers for the contents of distributionservices.

[0144] The radio base station 20 which provides the multicast servicesprocesses in accordance with a procedure shown in FIG. 14. Thisprocessing is mainly performed by the information distribution controlpart 24 of the radio base station 20.

[0145] The radio base station 20 enters one state of state of providingmulticast service and a state of not doing so. In FIG. 14, it isdetermined whether the radio base station 20 is performing multicastservice (S31). When the multicast service is performed (YES at S31), itis monitored whether it is a transmission timing for the multicastinformation channel table (see FIG. 13) which shows informationconcerning the multicast information channels under the service (S32),and, when it is the timing, the multicast information channel table isinformed of to all the radio terminals 10 in the service area Es (S33).

[0146] Then, it is determined whether a multicast service request signalis received from anyone of the radio terminals 10 (S34). When themulticast service request signal is received by none of the radioterminals 10 (NO at S34), the series of processing is finished. Afterthat, as long as the multicast service request signal from the radioterminal 10 is not received, processing (S31 through S34) describedabove is performed repeatedly. Consequently, each radio terminal 10 inthe service area Es can know the contents of services distributed at thetime, and the radio channels for receiving the contents of the services,by receiving the multicast information table transmitted from the radiobase station 20.

[0147] The above-mentioned transmission (S33) of the multicastinformation table is made by using a radio channel which can be used bythe radio base station 20. The radio channel which can be used should bea radio channel determined according to a channel allocation methodwhich the radio base station 20 employs. The radio channel used fortransmission of this multicast information channel table is informed ofto each radio terminal 10 present in the service area Es from the radiobase station 20 using a perch channel, for example. Each radio terminal10 receives the multicast information table from the radio base station20 using the informed radio channel.

[0148] In addition, it is also possible to transmit the multicastinformation channel table from the radio base station 20 periodically atfixed intervals. In this case, control information concerning thetransmission period can also be informed of by using the perch channel.

[0149] Moreover, the radio channel used for transmission of themulticast information channel table may also be fixed. In this case,when the radio channel known between the radio base station 20 and eachradio terminal 10 is previously fixed therebetween, it is not necessaryto include the information concerning the radio channel used fortransmission of the multicast information channel in the perch channel.

[0150] Returning to FIG. 14, in a state in which the radio base station20 does not perform multicast service (NO at S31), or when, in theabove-described process (S31 through S34), a multicast service requestsignal (including the above-mentioned multicast group address) isreceived by the radio base station 20 from the radio terminal whichnewly desires a service (YES at S34), the above-mentioned multicastinformation channel table (see FIG. 13) is referred to, and it isdetermined (S35) whether or not the service concerning the request hasbeen already provided. When the service has been already provided (YESat S35), the series of processing is finished, and, the above-describedprocessing (S31 through S34) is repeatedly performed, until a multicastservice request signal is received subsequently.

[0151] When the service concerning the request has not been provided yet(NO at S35), the multicast information channel table is updated by newlyregistering into the multicast information channel table the multicastgroup address which identifies the multicast service concerning therequest, the radio channel (transmission channel) used for the service,and an information channel therefor (S36). Then, the radio base station20 starts transmission of the multicast information corresponding to theservice concerning the above-mentioned request using the thus-determinedradio channel (S37). After that, the above-described processing (S31through S34) is performed repeatedly. In this process, the multicastinformation channel table updated as mentioned above is transmitted toeach radio terminal 10 present in the service area Es from the radiobase station 20 (S32, S33).

[0152] In the process in which the radio base station 20 performsprocessing in accordance with the abovedescribed procedure, and providesthe multicast services, each radio terminal 10 performs processing inaccordance with a procedure shown in FIG. 15.

[0153] In FIG. 15, the radio terminal 10 receives the perch channelfirst (S41). The radio base station 20 transmits, by using this perchchannel, various control information concerning the system, also,information indicating whether or not the radio base station 20 isproviding multicast service, and, in case of providing the multicastservice, transmits information concerning the radio channel used fortransmitting the multicast information channel table, transmissionperiod therefor, and so forth.

[0154] When reception of multicast service is desired by the radioterminal 10 (YES at S42), it is determined whether the own terminal hasalready received a certain multicast service from the radio base station20 (S43). When the own terminal has not received multicast service fromthe radio base station 20 yet (NO at S43), it is determined whether,based on the information informed of by the above-mentioned perchchannel, the radio base station 20 is providing multicast service (S44).When the radio base station 20 is providing multicast service (YES atS44), it is determined, based on the transmission period for themulticast information channel table informed of by the above-mentionedperch channel, whether or not it is a timing of receiving the multicastinformation channel table (S45). When it is the timing, the multicastinformation channel table transmitted from the radio base station 20will be received (S46) using the radio channel informed of by theabove-mentioned perch channel.

[0155] The radio terminal 10 determines whether or not the multicastinformation channel (multicast group) which the own terminal desires isregistered in the multicast information table, when the multicastinformation channel table is received thereby (S47). When this multicastinformation channel desired is registered there (YES at S47), i.e., whendistribution service of the desired multicast information has beenalready performed, the radio terminal 10 receives the distributioninformation (multicast information) of the multicast information channelusing the transmission channel corresponding to the desired multicastinformation channel in the multicast information channel table (S48).

[0156] In the above-mentioned process, when the radio terminal 10already receives some multicast service (YES at S43), while theabove-mentioned service is received thereby, the distributioninformation of the multicast information channel concerning the relevantnew request is received thereby by using the radio channel for receivingthe multicast information according to the above-described procedure(S45 through S48).

[0157] On the other hand, when the radio base station 20 is notproviding multicast service (NO at S44), or, when the radio base station20 is providing multicast service, but is not providing a service of thedesired multicast information channel (NO at S47), the radio terminal 10transmits a multicast service request signal for the desired multicastinformation channel to the radio base station 20 (S49), and waits forthe radio base station 20 to start the service of the desired multicastinformation channel. Then, when the multicast information channel tableupdated by this multicast service request signal (see S34 through S36 inFIG. 14) is received from the radio base station 20 (S46), the radioterminal 10 receives the distribution information of the new multicastinformation using the radio channel (transmission channel) specified bythe multicast information channel table (S47, S48).

[0158] As mentioned above, since the radio terminal 10 receives themulticast information channel table from the radio base station 20 bythe radio channel informed of by using the perch channel, even when theradio channel by which the multicast information channel table should betransmitted by the radio base station 20 changes, the radio terminal 10can always receive the multicast information channel table. Moreover,even when the transmission channel by which the information of themulticast information channel (multicast information) is transmitted bythe radio base station 20 is changed, since the change is reflected bythe multicast information channel table, the radio terminal 10 canreceive the distribution information of the desired multicastinformation channel by the radio channel specified in the relevantmulticast information channel table.

[0159] In the processing according to the procedure shown in FIG. 15,when the radio terminal 10 does not desire multicast service (NO atS42), the series of processing is finished. Consequently, the radioterminal 10 which does not particularly desire multicast servicecontinues the state of receiving the perch channel (S41, S42).

[0160] In the above-mentioned system, multicast information will bedistributed to the radio terminal 10 from the radio base station 20through the processing by the radio terminal 10 according to theprocedure shown in the processing of FIG. 15 and the processing by theradio base station 20 according to the procedure shown in FIG. 14, asshown in FIG. 16.

[0161] That is, the multicast information channel table is transmittedto the radio terminal using the transmission channel {circle over (1)}informed of by the perch channel, and, further, the distributioninformation of the multicast information channel (multicast group)registered in the multicast information channel table is transmitted tothe radio terminal 10 using the transmission channel {circle over (2)}registered in the multicast information channel table.

[0162] In the above system, during provision of the multicast service,when all the radio terminals that receive the multicast informationcorresponding to a multicast information channel in the service area Esstop desiring continuation of the relevant multicast service, the radiobase station 20 which provides the multicast service processes accordingto the procedure shown in FIG. 17, for example, in order to stop themulticast service.

[0163] In this processing, a service continuation check signalcorresponding to the flag state reset signal and a response signalcorresponding to the service continuation request signal, describedabove for the first embodiment, are transmitted and received between theradio base station 20 and the radio terminal 10.

[0164] In FIG. 17, it is determined whether it is a timing oftransmitting the service continuation check signal (S51). When it isdetermined that it is the transmitting timing for the servicecontinuation check signal, the service continuation check signal istransmitted to each radio terminal 10 present in the service area Esfrom the radio base station 20 (S52). Then, a timer at which a responsewaiting time is set starts operation (S53). In this state, it isdetermined whether or not the response signal in response to the servicecontinuation check signal is received from anyone of the radio terminals(S55), while it is determined whether or not the relevant timer hasreached a time-out state (S54). There, when the response signal isreceived from any one of the radio terminals before the time-out of theabove-mentioned timer, the radio base station 20 enters a state wheredistribution service of the multicast information channel concerning theresponse signal is continued (S56). Whenever it receives the responsesignal in response- to the service continuation check signal for eachservice from the radio terminal until the above-mentioned timer reachesthe time-out state, a state is entered where distribution service of themulticast information channel concerning the response signal iscontinued (S54, S55, S56).

[0165] On the other hand, when the above-mentioned timer reaches thetime-out state (YES at S54), distribution service of the multicastinformation channel for which no response signal is returned in responseto the service continuation check signal is terminated (S57). Then, therelevant multicast information channel table is updated by deleting theinformation concerning this terminated multicast information channelfrom the multicast information channel table (see FIG. 13) (S58).

[0166] Transmission of the above-mentioned service continuation checksignal by the radio base station 20 can be performed as follows.

[0167] For example, in the perch channel, the multicast group addressfor which continuation of service is to be checked can be described asthe service continuation check signal. In this case, the radio terminalreceives the perch channel for every transmission period of this servicecontinuation check signal.

[0168] Moreover, the service continuation check signal can betransmitted using the same transmission channel as the transmissionchannel of the multicast information channel table. In this case, theservice continuation check signal is transmitted at the same period asthe transmission period of the multicast information channel table.Moreover, as shown in FIG. 18; in this case, a flag as the servicecontinuation check signal can be included in the multicast informationchannel table. When the multicast information channel table istransmitted, the flag corresponding to the multicast group for which itis the timing for checking continuation of the service is set to “1”,and the flag corresponding to the multicast group for which it is notthe timing is reset into “0.” Thus, similar to the first embodiment, incase of transmitting to each radio terminal the multicast informationchannel table including the above-mentioned flag information as theservice continuation check signal, when each radio terminal continuesreception of the multicast information, it can be determined whether itis necessary to transmit the service continuation request signal, basedon the state of the flag information.

[0169] Furthermore, the service continuation check signal can also betransmitted by using a different radio channel independently from themulticast information table. In this case, the transmission period ofthe service continuation check signal can also be set independently, forexample, information of the transmission channel for this servicecontinuation check signal is given by using the perch channel.

[0170] Each radio terminal which receives the service continuation checksignal transmitted from the radio base station as mentioned aboveprocesses in accordance with a procedure shown in FIG. 19.

[0171] In FIG. 19, when the service check signal is received (YES atS61), in case continuation of the multicast service currently receivedis desired (YES at S62), the radio terminal starts a random timer inwhich a random time setting is possible (S63). Then, when the randomtimer by which the random time is set reaches a time-out state (YES atS64), a response signal is transmitted to the radio base station 20.

[0172] Even when the service continuation check signal transmitted fromthe radio base station 20 is a signal for a specific multicastinformation channel, even when it is a signal for all the multicastinformation channels under service by the radio base station 20, theidentifies (multicast group addresses, information channel numbers orthe like) for indicating all the multicast information channels forwhich service is desired are contained in the response signal.

[0173] Furthermore, when the radio terminal which has received themulticast service positively terminates reception of the service, itprocesses in accordance with a procedure shown in FIG. 20. That is,reception of the multicast information corresponding to the multicastinformation channel transmitted from the radio base station 20 isinterrupted (S71), and a service reception termination signal istransmitted to the radio base station 20 (S72). The identifiers(multicast group addresses, information channel numbers or the like)which indicate all the multicast information channels for whichreception of service be terminated are included in this servicereception termination signal.

[0174] The radio base station 20 which has received the servicereception termination signal from any one of the radio terminalstransmits a service continuation check signal for the multicastinformation channels indicated by the identifiers (multicast groupaddresses, information channel numbers or the like) contained in theservice reception termination signal, to each radio terminal present inthe service area Es, and determines the necessity of continuation of theservice in accordance with the procedure shown in FIG. 17. Then, when noresponse signal is received from any of the radio terminals, the radiobase station 20 terminates the service of these multicast informationchannels.

[0175] In the example described above, the control signals transmittedto the radio base station 20 from the radio terminal 10 are three, theservice request signal, response signal, and service receptiontermination signal. These signals can be transmitted by a random accesschannel which is used for accessing the radio base station 20 first, ata time of turning on of the power supply of the radio terminal, or byusing user data included in a packet or a control informationtransmission channel in case during uni-cast communication.

[0176] Moreover, in the above-described example, the time divisionmultiple access type (TDMA) is used as an access type for downlinksignals from the radio base station 20 to the radio terminal 10.However, a code division multiple access type (CDMA) is also applicablein the above-mentioned example, by a similar manner only by replacingthe identifiers by spread codes, etc.

[0177] In addition, also in this embodiment, the radio base station 20acts as an information distributing apparatus.

[0178] A third embodiment of the present invention will now bedescribed.

[0179] According to this embodiment, in case a mobile set used as aradio terminal moves across service areas (in case of handoff) whilereceiving multicast distribution of information, switching of basestation which acts as a distribution origin for the mobile set can bemade smoothly.

[0180] A system to which a multicast service providing method in thepresent embodiment is applied is configured as shown in FIG. 21.

[0181] The multicast service providing system shown in this figureincludes a radio base station 110 and a radio terminal 120. The radiobase station 110 transmits (distributes) information (multicast data)provided via a root network, such as an IP network, to the radioterminal 120 present in a radio area. The radio terminal 120 is, forexample, a cellular phone, or a personal computer terminal (PC terminal)which has a telecommunication function, the radio terminal 120 receivesmulticast data distributed from the radio base station, and when amovement arises, the radio base station acting as the other party ofcommunicative is rapidly switched.

[0182] The radio base station 110 has a network interface 111, a radiolink control device 112, a transmitting/receiving unit 113, an antenna114, a radio base station side multicast control device 115, a radiobase station side handoff control device 116, and an input/output device117.

[0183] The network interface 111 performs control concerning the thirdlayer of OSI reference model. Specifically, when TCP/IP is used as acommunication protocol, the network interface 111 performscomposition/decomposition of IP packets, and extraction/insertion ofmulticast addresses.

[0184] The radio link control device 112 performs control concerning thesecond layer of OSI reference model. Specifically, the radio linkcontrol device 112 performs composition/decomposition of radio frameswhich include data channels to which multicast data is inserted, andvarious control channels, in a combined manner.

[0185] The transmitting/receiving unit 113 performs control concerningthe first layer of OSI reference model. Specifically, thetransmitting/receiving unit 113 transmits and receives radio signals byconverting a baseband signal from the radio link control device 12 intoradio signal, and transmitting it through the antenna 114 or convertinga radio signal given through the antenna 114 into a base-band signal.

[0186] The radio base station side multicast control device 115 performscontrol for multicast cooperatively with a multicast control device ofthe radio terminal 120 described later. Moreover, the radio base stationside multicast control device 115 produces a multicast management tablewhich is information required in order that the radio terminal 120 canreceive multicast data. Details of the multicast management table willbe described later.

[0187] The radio base station side handoff control device 116 givesinstructions of distribution of multicast data to the radio base stationside multicast control device 115, and performs control of informing theradio terminal 120 of radio channels used for distributing the multicastdata to the radio terminal 120. Specifically, based on multicastaddresses obtained from the root network via the network interface,and/or information concerning link control obtained from the radio linkcontrol device 112, the radio base station side handoff control device116 performs handoff control for the radio terminal to switch the radiobase station acting as the other party of communication, and, also,gives radio channels used for the multicast to thetransmitting/receiving unit 113.

[0188] Moreover, the radio base station side handoff control device 116exchanges the multicast management table between the network interface111 and a radio base station adjacent thereto via the root networkconnected with the network interface 111, and in order to inform theradio terminal 120 of the multicast management tables of the own stationand the adjacent radio base station, it controls the radio link controldevice 112.

[0189] On the other hand, the radio terminal 120 has an antenna 121, atransmitting/receiving unit 122, a radio link control device 123, a userinterface 124, an input/output device 125, a radio terminal sidemulticast control device 126, and a radio terminal side handoff controldevice 127.

[0190] The transmitting/receiving unit 122 and the radio link controldevice 123 perform operation equivalent to the transmitting/receivingunit 113 and the radio link control device 112 of the radio base station110, respectively. However, the transmitting/receiving unit 122 alsoperforms measurement of electric field intensity of a signal transmittedfrom the radio base station 110.

[0191] The user interface 124 terminates the TCP/IP protocol for thesignal given from the radio link control device 123, and performsinterface with an input/output device 125. The input/output device 125outputs and inputs a sound, a picture, etc., and includes a display, aspeaker, a microphone, various keyboards, an external connectionterminals, etc.

[0192] The radio terminal side multicast control device 126 performscontrol for the multicast while communicating with the radio basestation side multicast control device 115 via the radio link controldevice 123 based on the information acquired through the user interface124.

[0193] The radio terminal side handoff control device 127 detects anopportunity of handoff, selects a radio base station to which a movementis made, performs request to the radio base station to which themovement is made for distribution of multicast data, and acquires themulticast management table.

[0194] Specifically, based on the information concerning electric fieldintensity obtained from the transmitting/receiving unit 122, and theinformation concerning the link control obtained from the radio linkcontrol device 123, the radio terminal side handoff control device 127performs synchronization with the radio base station side handoffcontrol device 116 and performs handoff. Moreover, the radio terminalside handoff control device 127 gives the radio channel used formulticast to the transmitting/receiving unit 122.

[0195] As embodiments realizing the above-mentioned functions will nowbe described. A first embodiment is such that the radio terminalrequests to the radio base station for which the radio terminal moves,for distribution of multicast data, directly, or, via the radio basestation at the movement origin, further acquires information foridentifying the radio channel used for the multicast, so that switchingof the radio base station which acts as the other party of communicationat the time of movement can be made rapidly. A second embodiment is suchthat the radio base station informs the radio terminal of the multicastmanagement table of the own station and also the multicast managementtable of an adjacent radio base station, the radio terminal previouslydetermines multicast data which the radio base station on the movementdestination distributes, and thus, the radio base station which acts asthe other party of communication can be switched rapidly at the time ofmovement.

[0196] First, the first example of operation will now be described. FIG.22(a) and (b) shows a conceptual diagram, and FIG. 23 shows an operationflow of the first example of operation. It is assumed that, as shown inFIG. 22(a), a radio terminal 120-1 in a radio zone Z1 of a radio basestation 110-1 on movement origin moves to a radio zone Z2 of a radiobase station 110-2 on movement destination, while receiving multicastdata given by the radio base station 110-1 on movement origin.

[0197] In this case, as shown in the operation flow of FIG. 23, when theradio terminal 120-1 moves into the radio zone Z2 from the radio zone Z1(S82) while receiving multicast data (S81), in the radio terminal 120-1,the signal reception intensity from the radio base station 110-2 onmovement destination becomes larger than the signal reception intensityfrom the radio base station 110-1 on movement origin. At this time, theradio terminal side handoff control device 127 built in the radioterminal 120-1 determines (S83), as an opportunity of handoff, the timeof the reception intensity of the perch channel from the radio basestation 110-2 on movement destination becoming larger than the receptionintensity of the perch channel from the radio base station 110-1 onmovement origin, based on the reception intensity of the perch channelreceived periodically.

[0198] In case of the opportunity of handoff, the radio terminal sidehandoff control device 127 transmits a switching request signal forswitching the radio base station to be communicated, from the radio basestation 110-1 on movement origin to the radio base station 110-2 onmovement destination, to the radio base station on movement destinationvia the radio link control device 123 (S84).

[0199] Information which indicates the radio base station 110-2 onmovement destination determined from the electric field intensity,information which indicates multicast data which the radio terminal120-1 desires to receive, information which indicates the radio terminal120-1 of transmission origin, and so forth, are included in theswitching request signal.

[0200] A random access channel which the radio base station 110-2 onmovement destination which is a transmission origin provides is used fortransmission of this switching request signal. Information concerningthe random access channel can be acquired from the perch channel fromthe radio base station 110-2 on movement destination.

[0201] The radio base station side handoff control device 116 built inthe radio base station 110-2 on movement destination receives theswitching request signal from the radio terminal 110-1, and, when themulticast data indicated by the switching request signal is notdistributed by the radio base station 110-2, it controls the radio basestation side multicast control device 115 to newly distribute themulticast data. The radio base station side multicast control device 115starts distribution of the multicast data which the radio terminal 120-1desires to receive, under such control (S85). When the multicast dataindicated by the switching request signal is already distributed by theradio base station 110-2, such control to the radio base station sidemulticast control device 115 by the radio base station side handoffcontrol device 116 is not performed.

[0202] Furthermore, the radio base station side handoff control device116 transmits, via the radio link control device 112, a switchingresponse signal including information which indicates multicast datadesired by the radio terminal 120-1 and started to be distributed oralready distributed, and information indicating a radio channel used fordistributing this multicast data, to the radio terminal 120-1 which hastransmitted the switching request signal (S86).

[0203] The radio terminal side handoff control device 127 built in theradio terminal 120-1 controls switching of the radio channel accordingto this switching response signal. Thereby, the radio terminal sidehandoff control device 127 starts reception of the multicast data fromthe radio base station 110-2 on movement destination (S87).

[0204] By the above-described series of operation, even when the radioterminal 120-1 moves into the radio zone Z2 of the radio base station110-2 on movement destination from the radio zone Z1 of the radio basestation 110-1 on movement origin, the radio base station to becommunicated with can be quickly switched.

[0205] In S84 of the flow of operation shown in FIG. 23, the radioterminal 120-1 may transmit the switching request signal to the radiobase station 110-1 on movement origin. Further, in order for the radiobase station 110-2 to increase opportunities to receive the switchingrequest signal so as to receive it positively, the radio terminal 120-1may transmit the switching request signal to both the radio base station110-1 on movement origin and radio base station 110-2 on movementdestination.

[0206] When transmitting the switching request signal to the radio basestation 110-1 on movement origin from the radio terminal, it isnecessary to include information which indicates the radio base station110-2 on movement destination in the switching request signal. In thiscase, when the radio base station 110-1 on movement origin receives theswitching request signal, the radio base station side handoff controldevice 116 built therein transfers the switching request signal to theradio base station 110-2 on movement destination via the networkinterface 110 and root network, according to the switching requestsignal. When the radio terminal 120-1 transmits the switching requestsignal to both the radio base station 110-1 on movement origin and theradio base station on movement destination, the radio base station 110-2on movement destination needs to select one of the two receivedswitching request signals. There is a method of selecting one having alower bit error rate between those two received switching requestsignals as a specific method of selection.

[0207] Further, in order for the radio base station to increaseopportunities to receive the switching request signal so as to receiveit positively, after the radio terminal 120-1 transmits the switchingrequest signal, when the switching response signal is not able to bereceived thereby even when a predetermined time has elapsed, theswitching request signal may be transmitted again. Further, the radioterminal 120-1 may previously transmits the switching request signalseveral times. Furthermore, the radio terminal 120-1 may transmit theswitching request signal several times, and, when the switching responsesignal is not able to be received thereby even when a predetermined timehas elapsed after the several times of transmission, the switchingrequest signal may transmitted again several times.

[0208] Thus, according to this example of operation, in case the radioterminal 120-1 moves across radio zones, by transmitting thedistribution request to the radio base station 110-2 on movementdestination, in response thereto, the radio base station 110-2 onmovement destination transmits the multicast data, and the switchingrequest signal including the information indicating this multicast dataand information indicating the radio channel used for distributing therelevant multicast data. Thereby, the radio base station to becommunicated with can be switch rapidly at the time of movement.

[0209] In this example of operation, when a plurality of radio terminalstransmit the switching request signals simultaneously, in order toprevent that these signals collide, the radio terminal knows a timing ofa multicast management table being informed of from the perch channel ofthe radio base station on movement destination, and, in case themulticast management table is inform of within a predetermined time, theradio terminal receives the multicast management table. Then, the radioterminal may transmit the switching request signal only when thecontents of the multicast management table do not include the multicastdata which it desires to continuously receive. On the other hand, incase the multicast data which it desires to receive continuously isincluded in the contents of the multicast management table, the radioterminal receives the multicast data in accordance with the contents,without transmitting the switching request signal.

[0210] Next, the second example of operation will now be described. Aconceptual diagram of the operation of the second example is shown inFIG. 24(a) and (b). Moreover, an example of a multicast management tableused in the operation example is shown in FIG. 25, and an example of anoperation flow thereof is shown in FIG. 26.

[0211] First, the multicast management table used in this embodimentwill now be described. In FIG. 24(a), the radio base station 110-1 onmovement origin receives information concerning multicast datadistributed by radio base stations 110-2 through 110-7 adjacent to theown station from the relevant radio base stations 110-2 through 110-7,and, informs to a radio zone Z1 at a period T₁ the multicast managementtable including this information and information concerning multicastdata distributed by the own station. The radio terminals 120-1 through120-3 receive this information transmitted from the radio base station110-1 on movement origin.

[0212] In the multicast management table shown in FIG. 25, the radiobase station number on first column is information which indicates theradio base stations 110-1 through 110-7. The radio base station numbers1 through 7 are related with the respective radio base stations 110-1through 110-7. The program name on second column is information whichindicates each multicast data. The IP address (multicast address)particular to a program name is related with each multicast data. In theexample of FIG. 25, the radio base station 110-1 distributes three sortsof multicast data, and the radio base station 110-7 distributes twosorts of multicast data.

[0213] The radio channel number on third column is informationidentifying radio channels, such as a career frequencies, radio slots,and spread codes, used for distribution of the multicast data identifiedby the program names on second column. The particular radio channelnumber is related with each radio channel.

[0214] The distribution check 1 on fourth column and the distributioncheck 2 on fifth column are used in order that the radio base station110 inquires for termination of distribution of multicast data to theradio terminals 120 present in the radio zone on the own station.

[0215] Specifically, when multicast data which becomes unnecessary todistribute is included in multicast data distributed from the radio basestation 110, the radio terminal 120 transmits a distribution unnecessarynotice including information which indicates the multicast data whichbecame unnecessary for the radio base station 110. In order to inquirefor permission of distribution termination to the radio terminals 120present in the radio zone on the own station, the radio base station 110which has received the distribution unnecessary notice informs themulticast management table in which the value on the distribution check1 for the multicast data indicated by the distribution unnecessarynotice is set as “1”, and the values on the distribution check 1 for theother multicast data are set as “0”.

[0216] When the value on the distribution check 1 for the multicast datawhich is under reception by itself is “1” in the received multicastmanagement table, the radio terminal 120 sets any value within apredetermined time in a random timer, and, at a timing at which therandom timer reaches the time-out state, transmits a response signalincluding information which indicates the multicast data under receptionto the radio base station 110.

[0217] When receiving the response signal within a predeterminedresponse interval, the radio base station 110 continues distribution ofthe multicast data corresponding to the response signal, i.e., continuesdistribution of the multicast data for which the value “1” is set on thedistribution check 1, but terminates the distribution when not receivingthe response signal therefor.

[0218] There is a case the radio base station 110 cannot receive thedistribution unnecessary notice from the radio terminal 120 due todegradation of propagation environment, power supply termination on theradio terminal, or the like. In this case, distribution of unnecessarymulticast data is continued and thus effective use of the radioresources cannot be attained. Therefore, the radio base station 110inquires for permission of distribution termination to the radioterminals 120 present in the radio zone for all the multicast datadistributed thereby.

[0219] Specifically, the radio base station 110 informs the multicastdata in which the value on the distribution check 2 is set as “1” atperiod Tq which is N times the period Tt of informing of the multicastmanagement table. When the value on the distribution check 2 of thereceived multicast data is “1” and, also, the multicast data is underreception thereby, the radio terminal 120 sets any value shorter than Tqin a random timer, and, at a timing at which the random timer reachesthe time-out state, transmits a response signal including informationindicating the multicast data which is under reception thereby.

[0220] The radio base station 110 continues distribution for themulticast data indicated by the response signal until the subsequentperiod Tq has elapsed. On the other hand, when the subsequent period Tqhas elapsed from the notice of the multicast data for which the value onthe distribution check 2 is set as “1”, the radio base station 10terminates distribution of the multicast data for which no indicationhas been made until then.

[0221] The multicast management table shown in FIG. 25 is produced bythe radio base station side handoff control device 116 built in theradio base station 110 periodically transmitting and receiving themulticast management table with the adjacent radio base stations 110 viathe network interface 11, and inserting in the multicast managementtable on the own station the thus-obtained multicast management tableson the adjacent radio base stations 110.

[0222] Next, specific operation will now be described. As shown in FIG.24(a), it is assumed that a radio terminal 120-1 in a radio zone Z1 of aradio base station 110-1 on movement origin moves to a radio zone Z1 ofa radio base station 110-2 on movement destination.

[0223] In this case, as shown in the operation flow of FIG. 26, whilethe radio terminal 120-1 receives the multicast data A and also receivesthe multicast management table at a period Tt (S91), it moves into theradio zone Z2 from the radio zone Z1 (S92), in the radio terminal 120-1,the signal reception intensity from the radio base station 110-2 onmovement destination becomes larger than the signal reception intensityfrom the radio base station 110-1 on movement origin. At this time, theradio terminal side handoff control device 127 built in the radioterminal 120-1 determines (S93), as an opportunity of handoff, the timeof the reception intensity of the perch channel from the radio basestation 110-2 on movement destination becoming larger than the receptionintensity of the perch channel from the radio base station 110-1 onmovement origin, based on the reception intensity of the perch channelreceived periodically.

[0224] In case of the opportunity of handoff, the radio terminal sidehandoff control device 127 refers to the latest multicast managementtable, and determines as to whether or not the multicast data A underreception thereby is distributed into the radio zone Z2 on the radiobase station 110-2 on movement destination.

[0225] When the multicast data A under reception thereby is notdistributed into the radio zone Z2 (NO at S94), the radio terminal sidehandoff control device 127 transmits a switching request signal forswitching the radio base station to be communicated from the radio basestation 110-1 on movement origin to the radio base station 110-2 onmovement destination to the radio base station on movement destinationvia the radio link control device 123 (S95). At this time, the radioterminal side handoff control device 127 may transmit the switchingrequest signal directly to the radio base station 110-2 on movementdestination, or to the radio base station 110-2 on movement destinationvia the radio base station 110-1 on movement origin.

[0226] As shown in FIG. 24(b), the radio base station side handoffcontrol device 116 built in the radio base station 110-2 on movementdestination receives the switching request signal from the radioterminal 110-1, and, controls the radio base station side multicastcontrol device 115 on the own station so as to distribute the multicastdata A, according to the switching request signal. Under the control,the radio base station side multicast control device 115 startsdistribution of the multicast data A (S96). Furthermore, the radio basestation side handoff control device 116 transmits a switching responsesignal to the radio terminal 120-1 which has transmitted the switchingrequest signal, via the radio link control device 112 (S97).

[0227] The radio terminal side handoff control device 127 built in theradio terminal 120-1 controls switching of the radio channel accordingto this switching response signal. Thereby, the radio terminal sidehandoff control device 127 starts reception of the multicast data A fromthe radio base station 110-2 on movement destination (S98).

[0228] Further, when the multicast data A under reception is distributedinto the radio zone Z2 (YES at S94), the radio terminal side handoffcontrol device 127 built in the radio terminal 120-1 controls switchingof the radio channel according to the multicast management table.Thereby, the radio terminal side handoff control device 127 startsreception of the multicast data A from the radio base station 110-2 onmovement destination (S99).

[0229] By the series of operation described above, even when the radioterminal 120-1 moves into the radio zone Z2 on the radio base station110-2 on movement destination from the radio zone Z1 on the radio basestation 110-1 on movement origin, the radio base station to becommunicated with can be rapidly switched.

[0230] Further, same as in the first embodiment, in S95 of the operationflow shown in FIG. 26, the radio terminal 120-1 may transmit theswitching request signal to the radio base station 110-1 on movementorigin. Further, the radio terminal 120-1 may transmit the switchingrequest signal to both the radio base station 110-1 on movement originand the radio base station 110-2 on movement destination.

[0231] Further, after the radio terminal 120-1 transmits the switchingrequest signal, when the switching response signal is not able to bereceived even when a predetermined time has elapsed, the switchingrequest signal may be transmitted again. Further, the radio terminal120-1 may previously transmits the switching request signal severaltimes. Furthermore, the radio terminal 120-1 may transmit the switchingrequest signal several times, and, when the switching response signal isnot able to be received even when a predetermined time has elapsed afterthe several times of transmission, the switching request signal maytransmitted again several times.

[0232] Thus, according to this example of operation, in case the radioterminal 120-1 moves across radio zones, information concerning themulticast data which the radio base station 110-2 on movementdestination distributes is included in the multicast management tableinformed of from the radio base station 110-1 on movement origin.Thereby, the radio terminal 120-1 can previously recognize the multicastdata which the radio base station 110-2 on movement destinationdistributes, and it receives the multicast data immediately withouttransmitting the distribution request, in case the radio base station110-2 on movement destination distributes the desired multicast data.Accordingly, the radio terminal 120-1 can rapidly switch the radio basestation to be communicated with at a time of movement.

[0233] Thus, according to the present invention, a radio terminal candetermine whether or not to transmit a service continuation requestsignal to an information distributing apparatus based on informationalindicating as to whether continuation request having been made formulticast information distribution service. Moreover, an informationdistributing apparatus can determine whether or not multicastinformation distribution service should be continued, based on amanagement state of service continuation management means. Thereby, theinformation distributing apparatus can provide multicast informationdistribution service properly via a radio section to radio terminalspresent within the service area.

[0234] Moreover, according to the present invention, radio channels usedfor multicast information distribution service can be limited only toradio channels which are informed of by an information distributingapparatus. Further, distribution services made by these radio channelscan be limited only to multicast information corresponding to the radiochannels informed of. Accordingly, it is possible to utilize the radioresources effectively, and, also, each radio terminal can receive onlythe necessary multicast information distribution services.

[0235] Furthermore, according to the present invention, in casenecessity arises for a radio terminal to switch the other party to becommunicated, from a radio base station on movement origin to a radiobase station on movement destination, information of a radio channelused for distributing multicast data is transmitted from the radio basestation on movement destination to the radio terminal, only as a resultof the radio terminal transmitting a distribution request to the radiobase station on movement destination. Thereby, it is possible to rapidlyswitch the radio base station to be communicated with at a time ofmovement, more rapidly than the prior art.

1. A multicast service providing system of performing multicastinformation distribution service to a radio terminal present within aservice area via a radio section from an information distributingapparatus, wherein: said radio terminal has service continuationrequesting means transmitting to said information distributing apparatusa service continuation request signal for requesting continuation ofmulticast information distribution service; said informationdistributing apparatus has service continuation management meansmanaging as to whether or not the above-mentioned request for servicecontinuation from the radio terminal for the multicast informationdistribution service has been made; and said information distributingapparatus transmits to said radio terminal information managed by theservice continuation management means has been made indicating whetheror not the continuation request for the multicast informationdistribution service has been made.
 2. The multicast service providingsystem as claimed in claim 1, wherein: said service management means hasmanagement state resetting means resetting a management state thereofinto a state that no request for continuation of service for themulticast information distribution service from the radio terminal hasbeen made.
 3. The multicast service providing system as claimed in claim2, wherein: said management state resetting means performs a resettingoperation for the management state every predetermined period.
 4. Themulticast service providing system as claimed in claim 2 or 3, wherein:said radio terminal has continuation request control means performing acontrol such that, when the information indicating whether or notcontinuation request for the multicast information distribution servicehas been made transmitted from the information distribution apparatusindicates a state that the request has not been made, the servicecontinuation requesting means transmits the service continuation requestsignal to the information distributing apparatus.
 5. The multicastservice providing system as claimed in claim 4, wherein: saidcontinuation request control means controls said service continuationrequesting means so that the service continuation request signal istransmitted after a first random time has elapsed since the informationindicating the state that no continuation request has been made for themulticast information distribution service was received from saidinformation distributing apparatus.
 6. The multicast service providingsystem as claimed in claim 4 or 5, wherein: said radio terminal hasre-transmission control means performing a control such that saidservice continuation requesting means re-transmits the servicecontinuation request signal to said information distributing apparatuswhen the information indicating as to whether or not continuationrequest has been made for the multicast information distribution servicehas not been changed into information indicating that the request hasbeen made within a predetermined time after the service continuationrequesting means transmitted the service continuation request signal. 7.The multicast service providing system as claimed in claim 6, wherein:said re-transmission control means controls said service continuationrequesting means so that the service continuation request signal istransmitted again after a second random time has elapsed after thepredetermined time had elapsed since said service continuationrequesting means transmitted the service continuation request signal. 8.The multicast service providing system as claimed in claim 6 or 7,wherein: a setting is made such that a time taken since theabove-mentioned management state resetting means reset the managementstate of the service request management means into a state that norequest for continuation of service for the multicast informationdistribution service has been made until the above-mentioned servicecontinuation request means re-transmits the service continuation requestsignal be shorter than a time taken since the above-mentioned managementstate was reset to the state that continuation for a service has notbeen made from the radio terminal for the multicast informationdistribution service until the resetting is made subsequently.
 9. Themulticast service providing system as claimed in any one of claims 1through 8, wherein: said service continuation management means managesas to whether or not continuation request has been made for themulticast information distribution service by using a flag which is madeto be ON when the service continuation request signal is received fromthe radio terminal.
 10. A multicast service providing method, wherein:when an information distributing apparatus performs multicastinformation distribution service to a radio terminal present within aservice area via a radio section, said information distributingapparatus manages as to whether or not request for service continuationfrom the radio terminal for the multicast information distributionservice has been made, and, also, transmits thus-managed informationindicating whether or not the continuation request for the multicastinformation distribution service management has been made to the radioterminals, and said radio terminal transmits a service continuationrequest signal to said information distributing apparatus when theinformation transmitted from the information distributing apparatusindicating whether or not service continuation request has been made bysaid radio terminal for the multicast information distribution serviceindicates a state that the request has not been made.
 11. The multicastservice providing method as claimed in claim 10, wherein: the managementstate as to whether or not continuation request has been made for themulticast information distribution service from the radio terminal isreset to a state that no continuation request for the multicastinformation distribution service has been made from the radio terminal,periodically.
 12. The multicast service providing method as claimed inclaim 10 or 11, wherein: when the information indicating whether or notcontinuation request has been made for the multicast informationdistribution service is not changed into a state that the request hasbeen made within a predetermined time after the radio terminaltransmitted the service continuation request signal, the servicecontinuation request signal is transmitted to the informationdistributing apparatus again.
 13. An information distributing apparatuswhich performs multicast information distribution service via a radiosection to a radio terminal present within a service area, comprising:service continuation management means managing as to whether or notservice continuation request has been made by the radio terminal for themulticast information distribution service, information managed by saidservice continuation management means indicating as to whether or notcontinuation request for the multicast information distribution servicebeing transmitted to the radio terminal.
 14. The informationdistributing apparatus as claimed in claim 13, wherein: the servicecontinuation management means has management state resetting meansresetting the management state into a state that no service continuationrequest has been made by the radio terminal for the multicastinformation distribution service.
 15. The information distributingapparatus as claimed in claim 14, wherein: said management stateresetting means performs the resetting operation every predeterminedperiod.
 16. A radio terminal which receives multicast informationdistribution service from an information distributing apparatus via aradio section, comprising: service continuation request meanstransmitting a service continuation request signal for requestingprovision of the multicast information distribution service, to theinformation distributing apparatus; and continuation request controlmeans controlling said service continuation request means so as to causeit to transmit the service continuation request signal to theinformation distributing apparatus when information indicating whetheror not service continuation request has been made by the radio terminalfor the multicast information distribution service, transmitted by theinformation distributing apparatus which manages as to whether or notthe service continuation request has been made by the radio terminal forthe multicast information distribution service, indicates a state thatthe request has not been made.
 17. The radio terminal as claimed inclaim 16, comprising re-transmission control means controlling theservice continuation request means so that the service continuationrequest means re-transmits the service continuation request signal tothe information distributing apparatus, when the information indicatingwhether or not service continuation request has been made by the radioterminal for the multicast information distribution service has not beenchanged into a state that the request has been made within apredetermined time after the service continuation request meanstransmitted the service continuation request signal.
 18. A multicastservice providing method by which multicast information distributionservice is performed by an information distributing apparatus via aradio section on radio terminals present in a service area thereof:wherein: the information distributing apparatus informs all the radioterminals present in the service area of information for identifyingmulticast information on distribution and radio channels used for thedistribution of the multicast information by using a predetermined radiochannel, and the respective radio terminals within the service areareceive the multicast information distribution service from theinformation distributing apparatus via the thus-informed radio channels.19. The multicast service providing method as claimed in claim 18,wherein: the information distributing apparatus manages correspondencerelationship between the information for identifying the multicastinformation on distribution service and the radio channels used fordistributing the multicast information, and informs all the radioterminals present within the service area management informationindicating the correspondence relationship by using the above-mentionedpredetermined radio channel, and each of the radio terminals presentwithin the service area receives the desired multicast informationdistribution service by using the corresponding radio channels based onthe management information.
 20. The multicast service providing methodas claimed in claim 19, wherein: the radio terminal transmits a servicerequest signal for requesting distribution service of multicastinformation not included in the management information, to theinformation distributing apparatus; when receiving said service requestsignal from the radio terminal, the information distributing apparatusadds to the management information a correspondence relationship betweeninformation identifying the multicast information concerning saidrequest and a radio channel used for distributing the multicastinformation; and, also starts distribution service of the multicastinformation concerning said request by using said radio channel.
 21. Themulticast service providing method as claimed in any one of claims 18through 20, wherein: the information distributing apparatus informs acontinuation check signal for inquiring a necessity of continuation ofthe multicast information distribution service, to all the radioterminals present in the service area; when receiving the continuationcheck signal from the information distributing apparatus, the radioterminal transmits a predetermined response signal to the informationdistributing apparatus for continuously receiving the multicastinformation distribution service; and when receiving the predeterminedresponse signal after transmitting the continuation check signal, theinformation distributing apparatus continues distributing service of themulticast information concerning the inquiry.
 22. The multicast serviceproviding method as claimed in claim 21, wherein: in case the responsesignal has not been received from any of the radio terminals in responseto the continuation check signal within a predetermined time, theinformation distributing apparatus terminates distribution service ofthe multicast information concerning the inquiry.
 23. The multicastservice providing method as claimed in claim 21 or 22, said informationdistributing apparatus manages correspondence relationship betweeninformation identifying the multicast information on distributionservice and radio channels used for distributing the multicastinformation, and transmits management information indicating thecorrespondence relationship to all the radio terminals present withinthe service area; and a correspondence relationship between multicastinformation for which distribution service is terminated and a radiochannel used for distributing the multicast information is deleted fromthe management information.
 24. The multicast service providing methodas claimed in claim 21 or 22, wherein: said information distributingapparatus manages correspondence relationship between information foridentifying multicast information on distribution service and radiochannels used for distributing the multicast information, and informsmanagement information indicating the correspondence relationship to allthe radio terminals present within the service area by using thepredetermined channel; and deletes a correspondence relationship betweeninformation for identifying multicast information for which distributionservice is terminated and a radio channel used for distributing themulticast information from the management information.
 25. The multicastservice providing method as claimed in any one of claims 21 through 24,wherein: the radio terminal transmits a reception termination signal tothe information distributing apparatus for terminating distributionservice of multicast information; and when receiving the receptiontermination signal from the radio terminal, the information distributingapparatus informs a continuation check signal for inquiring a necessityof continuation of the multicast information distribution serviceconcerning the distribution service reception termination to all theradio terminals present within the service area.
 26. A multicast serviceproviding system in which multicast information distribution service ismade by an information distributing apparatus to radio terminals presentwithin a service area via a radio section, wherein: the informationdistributing apparatus has information control means informing, by usinga predetermined radio channel, all the radio terminals present withinthe service area, of information for identifying multicast informationon distribution service and radio channels used for distributing themulticast information; and each radio terminal present within theservice area receives the multicast information distribution servicefrom the information distributing apparatus by using the radio channelsthus informed of.
 27. The multicast service providing system as claimedin claim 26, wherein: the information distributing apparatus hasmanagement means managing correspondence relationship between theinformation identifying the multicast information on distributionservice and the radio channels used for distributing the multicastinformation, and informs, of management information indicating thecorrespondence relationship, all the radio terminals present within theservice area by using a predetermined radio channel; and each radioterminal present within the service area receives the desired multicastinformation distribution service from the information distributingapparatus via the corresponding radio channels based on the managementinformation.
 28. The multicast service providing system as claimed inclaim 27, wherein: the radio terminal has service requesting meanstransmitting a service request signal for requesting distributionservice for multicast information not included in the managementinformation; the information distributing apparatus has first managementinformation updating means adding, when receiving said service requestsignal from the radio terminal, to the management information, acorrespondence relationship between information for identifying themulticast information concerning the request and a radio channel usedfor distributing the multicast information, and as well as updates themanagement information by the first management information updatingmeans, starts distribution service of the multicast informationconcerning the request by using said radio channel.
 30. The multicastservice providing system as claimed in any one of claims 27 through 29,wherein: the information distributing apparatus has service continuationcheck means informing all the radio terminals present within the servicearea of a continuation check signal for inquiring a necessity ofcontinuation of the multicast information distribution service; theradio terminal has response signal transmission control meanstransmitting, when receiving the continuation check signal from theinformation distributing apparatus, a predetermined response signal forcontinuously receiving the multicast information distribution service;and, further the information distributing apparatus has servicecontinuation control means continuing, when receiving the predeterminedresponse signal from the service continuation check means aftertransmitting the service continuation check signal, the multicastinformation distribution service concerning the inquiry.
 31. Themulticast service providing system as claimed in claim 30, wherein: theinformation distributing apparatus has service termination control meansterminating, when the response signal has not been received from any ofthe radio terminals within a predetermined time in response to thecontinuation check signal, the multicast information distributionservice concerning the inquiry.
 32. The multicast service providingsystem as claimed in claim 30 or 31, wherein: said service continuationcheck means informs all the radio terminals present within the servicearea of the continuation check signal as well as information foridentifying the multicast information on distribution service and radiochannels used for distributing the multicast information by using thepredetermined radio channel.
 33. The multicast service providing systemas claimed in any one of claims 30 through 32, wherein: said informationdistributing apparatus manages correspondence relationship betweeninformation identifying multicast information on distribution serviceand radio channels used for distributing the multicast information, andinforms all the radio terminals present within the service areamanagement information indicating the correspondence relationship byusing the predetermined radio channel; and said information distributingapparatus has second management information updating means deleting acorrespondence relationship between information identifying multicastinformation for which distribution service is terminated and a radiochannel used for distributing the multicast information, from themanagement information.
 34. The multicast service providing system asclaimed in any one of claims 30 through 32, wherein: the radio terminalhas termination control means transmitting, when terminating receptionof a multicast information distribution service, a reception terminationsignal to the information distributing apparatus; and when receiving thereception termination signal from the radio terminal, the servicecontinuation check means of the information distributing apparatustransmits a continuation check signal for inquiring a necessity ofcontinuation of the multicast information distribution serviceconcerning the distribution service reception termination to all theradio terminals present within the service area.
 35. An informationdistributing apparatus performing multicast information distributionservice for radio terminals present within a service area via a radiosection, comprising: informing control means for informing all the radioterminals present within the service area of information for identifyingmulticast information on distribution service and radio channels usedfor distributing the multicast information by using a predeterminedradio channel, each of the radio terminals present within the servicearea being able to receive the multicast information distributionservice via the thus-informed radio channels.
 36. The informationdistributing apparatus claimed in claim 35, comprising: management meansmanaging correspondence relationship between multicast information ondistribution service and radio channels used for distributing themulticast information, said informing control means informing all theradio terminals present within the service area of managementinformation indicating the correspondence relationship managed by saidmanagement means, by using the predetermined radio channel.
 37. Theinformation distributing apparatus as claimed in claim 36, wherein: saidmanagement means comprises first management information updating meansadding, when receiving a service request signal concerning multicastinformation for which distribution service has not been made from theradio terminal, a correspondence relationship between informationidentifying the multicast information concerning the request managed bysaid management means and a radio channel used for distributing themulticast information, to the management information, as well as themanagement information being updated by said first managementinformation updating means, distribution service of the multicastinformation concerning the request being started by using said radiochannel.
 38. The information distributing apparatus as claimed in anyone of claims 35 through 37, comprising: service continuation checkmeans informing all the radio terminals present within the service areaof a continuation check signal for inquiring a necessity of continuationof the multicast information distribution service; and servicecontinuation control means continuing, when receiving a predeterminedresponse signal after transmitting the service continuation check signalby said service continuation check means, the multicast informationdistribution service concerning the inquiry.
 39. The informationdistributing apparatus as claimed in claim 38, comprising: servicetermination control means terminating, when a response signal has notbeen received from any one of the radio terminals within a predeterminedtime in response to the continuation check signal by said servicecontinuation check means, the multicast information distribution serviceconcerning the inquiry.
 40. The information distributing apparatus asclaimed in claim 38 or 39, wherein: said service continuation checkmeans informs all the radio terminals present within the service area ofthe continuation check signal as well as information for identifying themulticast information on distribution service and radio channels usedfor distributing the multicast information by using the predeterminedradio channel.
 41. The information distributing apparatus as claimed inany one of claims 38 through 40, comprising: said management meansmanaging correspondence relationship between information identifyingmulticast information on distribution service and radio channels usedfor distributing the multicast information, and informing all the radioterminals present within the service area management information managedby said management means indicating the correspondence relationship byusing the predetermined radio channel; and second management informationupdating means deleting a correspondence relationship betweeninformation identifying multicast information for which distributionservice is terminated and a radio channel used for distributing themulticast information, from the management information.
 42. Thedistributing apparatus as claimed in any one of claims 38 through 41,wherein: when receiving, from the radio terminal receiving multicastinformation distribution service, a reception termination signalindicating termination of the reception of distribution service, saidservice continuation check means transmits a continuation check signalfor inquiring a necessity of continuation of the multicast informationdistribution service concerning the distribution service receptiontermination to all the radio terminals present within the service area.43. A radio terminal which receives multicast information distributionservice via a radio section from an information distributing apparatus,comprising: service state reception control means receiving from theinformation distributing apparatus management information indicatingcorrespondence relationship between information for identifyingmulticast information on distribution service and radio channels usedfor distributing the multicast information, such as to receive from theinformation distributing apparatus the desired multicast informationdistribution service via the corresponding radio channels based on themanagement information received by the service state reception controlmeans.
 44. The radio terminal as claimed in claim 43, comprising:service requesting means transmitting a service request signal forrequesting a distribution service for multicast information not includedin the management information to the information distributing apparatus,such that the information distributing apparatus may add, when receivingsaid service request signal from the radio terminal, to the managementinformation, a correspondence relationship between information foridentifying the multicast information concerning the request and a radiochannel used for distributing the multicast information, and, also,distribution service of the multicast information concerning the requestmay be started by using said radio channel.
 45. The radio terminal asclaimed in claim 43 or 44, comprising: response signal transmissioncontrol means transmitting, when receiving a continuation check signalfor inquiring a necessity of continuation of multicast informationdistribution service from the information distributing apparatus, apredetermined response signal for continuously receiving the multicastinformation distribution service.
 46. The radio terminal as claimed inclaim 45, comprising: termination control means transmitting a receptiontermination signal to the information distributing apparatus forterminating reception of multicast information distribution service,such that said information distributing apparatus may inform, whenreceiving the reception termination signal form the radio terminal, allthe radio terminals present within the service area of a continuationcheck signal for inquiring a necessity of continuation of the multicastinformation distribution service concerning the distribution servicereception termination.
 47. A multicast service providing method by whichmulticast data is distributed to radio terminals from a radio basestation, wherein: the radio terminal which receives the multicast datatransmits a distribution request for multicast data to a radio basestation on movement destination in case a necessity arises such that theother party of communication be switched from a radio base station onmovement origin to the radio base station on movement destination, theradio base station on movement destination transmits information of aradio channel used for distributing the multicast data to the radioterminal in response to the distribution request given therefrom, andthe radio terminal switches the radio channel based on the informationof the radio channel given by the radio base station on movementdestination and receives the multicast data from the radio base stationon movement destination.
 48. The multicast service providing method asclaimed in claim 47, wherein: the-radio terminal transmits thedistribution request for the multicast data to the radio base station onmovement origin, and the distribution request for the multicast data istransmitted to the radio base station on movement destination via theradio base station on movement origin.
 49. The multicast serviceproviding method as claimed in claim 47 or 48, wherein: in case theradio terminal cannot receive information of the radio channel to beused for distributing the multicast data within a predetermined timeafter transmitting the distribution request, the radio terminaltransmits the distributing request again.
 50. The multicast serviceproviding method as claimed in any one of claims 47 through 49, wherein:the radio terminal transmits the distribution request several times. 51.The multicast service providing method as claimed in any one of claims47 through 50, wherein: the radio base station on movement originobtains information concerning multicast data distributed by an adjacentradio base station, and transmits the information concerning themulticast data, and in case the radio terminal determines that the radiobase station on movement destination does not distribute the desiredmulticast data based on the information concerning the multicast datagiven by the radio base station on movement origin, the radio terminaltransmits the distribution request to the radio base station on movementdestination or the radio base station on movement origin so that thedistribution request may be received by the radio base station onmovement destination.
 51. A radio base station which transmits multicastdata to a radio terminal, comprising: distribution request receivingmeans receiving a distribution request for multicast data transmittedfrom the radio terminal, and radio channel information transmittingmeans transmitting information of a radio channel used for distributingthe multicast data in response to the distributing request for themulticast data given by the radio terminal.
 52. The radio base stationas claimed in claim 51, wherein: the distribution request receivingmeans receives the distribution request for multicast data transmittedfrom the radio terminal via another radio base station.
 53. The radiobase station as claimed in claim 51 or 52, wherein: the distributionrequest receiving means comprises information obtaining means obtaininginformation concerning multicast data transmitted from a radio basestation adjacent thereto, and multicast information transmitting meanstransmitting the information concerning the multicast information.
 54. Aradio terminal which receives multicast data distributed by a radio basestation, comprising: distribution request transmitting meanstransmitting a distribution request for multicast data to a radio basestation on movement destination in case a necessity arises such that theother party of communication be switched from a radio base station onmovement origin to the radio base station on movement destination, andmulticast data reception control means for switching a radio channelbased on information concerning a radio channel used for distributingthe multicast data transmitted from the radio base station on movementdestination in response to the distribution request so as to receive themulticast data distributed from the radio base station on movementdestination.
 55. The radio terminal as claimed in claim 54, wherein: thedistribution request transmitting means transmits the distributionrequest for the multicast data to the radio base station on movementorigin so that the distribution request for the multicast data betransmitted to the radio base station on movement destination via theradio base station on movement origin.
 56. The radio terminal as claimedin claim 54 or 55, wherein: the request transmitting means transmits thedistribution request again in case information concerning the radiochannel to be used for distributing the multicast data cannot bereceived within a predetermined time after transmitting the distributingrequest.
 57. The radio terminal as claimed in any one of claims 54through 56, wherein: the distribution request transmitting meanstransmits the distribution request several times.
 58. The radio terminalas claimed in any one of claims 54 through 57, wherein: the distributionrequest transmitting means transmits the distribution request to theradio base station on movement destination or the radio base station ondestination origin so that the distribution request for the multicastdata be received by the radio base station on movement destination incase the distribution request transmitting means determines that theradio base station on movement destination does not distribute thedesired multicast data based on information, received from the radiobase station on movement origin, concerning multicast informationdistributed from the radio base station adjacent to the station.